The phrase person from Porlock and its variants denote a person who interrupts at an inconvenient moment.

This phrase alludes to a visitor from Porlock, in Somerset, England, who, according to the English poet, critic and philosopher Samuel Taylor Coleridge (1772-1834), interrupted him during the composition of Kubla Kahn. Coleridge mentioned the incident in Of the Fragment of Kubla Khan, the preface to Kubla Khan: Or a Vision in a Dream, published in Christabel: Kubla Khan, A Vision: The Pains of Sleep (London: Printed for John Murray, 1816):

In the summer of the year 1797, the Author, then in ill health, had retired to a lonely farm house between Porlock and Linton, on the Exmoor confines of Somerset and Devonshire. In consequence of a slight indisposition, an anodyne had been prescribed, from the effects of which he fell asleep in his chair at the moment that he was reading the following sentence, or words of the same substance, in “Purchas’s Pilgrimage:” “Here the Khan Kubla commanded a palace to be built, and a stately garden thereunto. And thus ten miles of fertile ground were inclosed with a wall.” The author continued for about three hours in a profound sleep, at least of the external senses, during which time he has the most vivid confidence, that he could not have composed less than from two to three hundred lines; if that indeed can be called composition in which all the images rose up before him as things, with a parallel production of the correspondent expressions, without any sensation or consciousness of effort. On awaking he appeared to himself to have a distinct recollection of the whole, and taking his pen, ink, and paper, instantly and eagerly wrote down the lines that are here preserved. At this moment he was unfortunately called out by a person on business from Porlock, and detained by him above an hour, and on his return to his room, found to his no small surprise and mortification, that though he still retained some vague and dim recollection of the general purpose of the vision, yet, with the exception of some eight or ten scattered lines and images, all the rest had passed away like the images on the surface of a stream into which a stone has been cast, but, alas! without the after restoration of the latter:
Then all the charm
Is broken—all that phantom-world so fair
Vanishes, and a thousand circlets spread,
And each mis-shape the other. Stay awhile,
Poor youth! who scarcely dar’st lift up thine eyes—
The stream will soon renew its smoothness, soon
The visions will return! And lo, he stays,
And soon the fragments dim of lovely forms
Come trembling back, unite, and now once more
The pool becomes a mirror.

The visitor from Porlock who, according to Coleridge, interrupted the composition of Kubla Khan was mentioned in the following from Instructions in Cottage Cookery, published in The West Somerset Free Press (Williton, Somerset, England) of 21^st September 1878:

The districts of Minehead and Porlock are notable in many ways. They witness the last survival of something like real stag-hunting in England; they contain some of the most delightful scenery even of the delightful West country, and they were once the sojourn of Wordsworth ¹ and Coleridge. It was a “person from Porlock” who is branded to all time with the crime of having interrupted and for ever put a stop to the composition or rather reminiscence of Kubla Khan; it was in the same neighbourhood that Shelvocke’s voyages gave to one poet or the other the idea of the Ancient Mariner ²; and here, too, arrived that remarkable Government spy who was so difficultly convinced of Coleridge’s political innocence, because of his suspicious and apparently personal reference to “Spy-nosy,” that is to say, Spinosa.

¹ William Wordsworth (1770-1850) was an English poet.
² The episode that lies at the heart of Samuel Taylor Coleridge’s The Rime of the Ancient Mariner – the killing of an albatross – was suggested by William Wordsworth, who had recently read A Voyage Round the World by the Way of the Great South Sea (London: Printed for J. Senex, W. and J. Innys, and J. Osborn and T. Longman, 1726), by the English Royal-Navy officer and later privateer George Shelvocke (c1675-1742): on 1^st October 1719, during a semi-authorised pirate expedition aboard The Speedwell in the South Seas, a black albatross that was following the ship had been shot by Simon Hatley, Shelvocke’s second captain.

In He and She; Or, A Poet’s Portfolio (Boston, Massachusetts: Houghton, Mifflin and Company, 1884), by the U.S. sculptor, art critic and poet William Wetmore Story (1819-1895), the phrase person from Porlock refers explicitly to Coleridge:

He. It is strange how at the very culmination of exalted feeling, when the sensibilities are all alive, fate seems to take a special pleasure in doing them some prosaic violence. How the commonplace and even contemptible facts of life will rush in athwart us in our most poetic moods, and compel us to laugh, despite our annoyance. The lover is just declaring his passion to some trembling girl, for instance, when Bridget opens the door to say, “Please Miss, the butcher says shall he leave a leg of mutton, or will you have a pair of chickens;”—or just as the poet is in the height, let us call it, of his inspiration, some “person from Porlock” will come in on business matters, to try on one’s new shoes, perhaps, and the vision of Kubla Khan disappears beyond the horizon of recovery.
She. It is lucky that the “person from Porlock” was anonymous, or hundreds of us would have taken his life.
He. I wonder if he ever existed. It would be just like Coleridge to have invented him as an excuse for his own laziness.
She. Whether he existed or not, he exists no longer, so let us think no more of him, since both he and Coleridge have gone beyond recall, and no one can ever finish that exquisite fragment which he interrupted.

The phrase person from Porlock is used with implicit reference to Coleridge in the review of an essay ³, published in The Saturday Review of Politics, Literature, Science, and Art (London, England) of 6^th October 1888:

The theme of a wondrous moral essay is supplied by Mr. Browning ⁴ and Buffalo Bill ⁵, of which the only novelty lies in the conjunction. Many excellent people are strangely exercised by the problem “How to read Mr. Browning.” These aspirants to culture have made some progress of late, or the Browning Society has laboured in vain. But in the far and boundless West, where Buffalo Bill’s Indians once roamed free in “reserves,” they have altogether shamed our blushing endeavours. Our Browning Society is but a poor shade of their Browning Clubs. The cult has become occult. In one Western State there is a “Browning Club ” that does not desire to be known as such, or rather did not till some inquisitive person, like the “person from Porlock,” broke the spell.

³ This essay was published in Harper’s New Monthly Magazine (New York City, New York, USA) of October 1888.
⁴ Robert Browning (1812-1889) was an English poet.
⁵ Buffalo Bill (William Frederick Cody – 1846-1917) was a U.S. showman.

The following is from The Saturday Review of Politics, Literature, Science, and Art (London, England) of 8^th December 1888:

PERSONS FROM PORLOCK.

Can nothing be done to protect poets?—to compensate them, and the world, for their lost lays is impossible. Can nothing be done to preserve them from the visits of Persons from Porlock? It may be remembered, or it may be forgotten, that a Person from Porlock, on business, interrupted Mr. Coleridge as he was writing out the poem of Kubla Khan, which he had composed in a vision. In a vision once he saw a damsel, with a dulcimer, singing of Mount Abora. It was Mr. Coleridge’s deliberate opinion that, could he revive within him her symphony and song, he would be enabled to build that dome in air, that sunny dome, those caves of ice; and that all would cry, “Beware, beware,” and invite the general public to weave a circle round him thrice. These adventures have been rendered impossible by the visit of the Person from Porlock, on business, and have ever been regretted by sensible hearts.

And the following is from The Wedded Poets, by Mrs. Andrew Crosse, published in Temple Bar: A London Magazine for Town and Country Readers (London, England) of January 1892:

The ben trovato story of the publisher’s answer to the “person from Porlock,” or elsewhere, who desired to sell his volume of verse for a good round sum, and see himself famous, may be recalled; the man of business in rejecting the obliging offer of the unknown, observed, “There is no market for poetry at present; if Shakespeare were alive, he would have difficulty in finding a publisher; indeed, I will go further—if Prince Albert himself were to offer a volume of poems, it would probably be declined.”