The verb unfriend was coined by the Church of England clergyman Thomas Fuller (1608-61) in The Appeal of Injured Innocence (1659). Writing to Peter Heylin (1599-1662), a churchman who had criticised The Church History of Britain from the birth of Jesus Christ until the year 1648, published in 1655, Fuller declared:

I hope, Sir, that we are not mutually un-friended by this difference which hath happened betwixt us. And now, as duellers, when they are both out of breath, may stand still and parley, before they have a second pass, let us in cold blood exchange a word, and, mean time, let us depose, at least, suspend, our animosities.
[…] I conceive our time, pains, and parts may be better expended to God’s glory, and the Church’s good, than in these needless contentions. Why should Peter fall out with Thomas, both being disciples to the same Lord and Master? […]
Who knoweth but that God, in his providence, permitted, yea, ordered, this difference to happen betwixt us, not only to occasion a reconciliation, but to consolidate a mutual friendship betwixt us during our lives, and that the survivor (in God’s pleasure only to appoint) may make favourable and respectful mention of him who goeth first to his grave?

But this was in fact the only use of the verb until it was coined again in the early 2000s to mean to remove (someone) from a list of friends or contacts on a social networking website (defriend has also been coined in that specific sense). Jeremy Butterfield, editor of the 2015 edition of Fowler’s Dictionary of Modern English Usage, remarks:

There can be no doubt that Fuller would have been perplexed by the transient, trivial nature of ‘friendships’ that can be terminated with a keystroke.—“I wrote ‘your’ instead of ‘you’re’ on a post and she corrected that so I unfriended her.” (Sunday Times, 2012)

Thomas Fuller invented a number of other rare or nonce verbs prefixed by un-. For example:

– In The history of the worthies of England (1662), about Dorset:

Thomas Winniffe was born at Sherborne in this County, and was bred contemporary with Doctor Hackwell in Exeter Colledge in Oxford, and we may observe a three-fold parallel betwixt these two eminent persons. First they were Fellows of the same foundation. Secondly Chaplains to the same illustrious Master, Prince Henry. Thirdly, both out of (indiscretion at the worst) no ill intent, ran on the same Rock, though not to the same degree of damage. Dr. Hackwel, for opposing the Spanish Match, was un-Chaplain’d, and banished the Court; Doctor Winniffe, for a passage in his Sermon (not against, but) about Gondomer, was committed close prisoner to the Tower, and there for some days remained.

– In The holy state (1642), about Cesar Borgia:

His Father first made him a Cardinall, that thereby his shoulders might be enabled to bear as much Church-preferment as he could load upon him. But Borgia’s active spirit disliked the profession, and was ashamed of the Gospel, which had more cause to be ashamed of him; wherefore he quickly got a dispensation to uncardinall himself.

While the verb unfriend was not in use, the adjective unfriended was, from the mid-16^th to the 19^th centuries. For example, the English playwright and poet William Shakespeare (1564-1616), in Twelfe Night, Or what you will (around 1601), makes Antonio say to Sebastian:

(Folio 1, 1623)
I could not stay behinde you: my desire
(More sharpe then filed steele) did spurre me forth,
And not all loue to see you (though so much
As might haue drawne one to a longer voyage)
But iealousie, what might befall your trauell,
Being skillesse in these parts: which to a stranger,
Vnguided, and vnfriended, often proue
Rough, and vnhospitable. My willing loue,
The rather by these arguments of feare
Set forth in your pursuite.

The noun unfriend, meaning an enemy, or one who is not a friend or on friendly terms, is first recorded in Brut (late 12^th-early 13^th century), a poetical paraphrase by the poet Laȝamon of The Roman de Brut (mid-12^th century), by the Norman poet Wace:

And we sollen wende; and wiþ ham fihten.
slean houre onfrendes; and wenden after Brenne.
     translation:
And we shall march, and with them fight;
Slay our unfriends, and proceed after Brennes.
[…]
Vther bead ferde; ouer al þeos eorþe.
and wend to oure onfreondes; and drif heom of londe.
     translation:
Uther, summon forces over all this land,
And march to our unfriends and drive them from land.

This noun went out of use in the 17^th century and was briefly revived by the Scottish novelist and poet Walter Scott (1771-1832) in Waverley; or, ’Tis Sixty Years Since (1814):

He is a very unquiet neighbour to his un-friends.