Tom Pepper is the name of a character proverbially said to have been so great a liar that he was expelled from Hell.

Hence, frequently in the phrase a bigger liar than Tom Pepper, and variants, Tom Pepper designates an outrageous liar.

The Oxford English Dictionary (OED – 3^rd edition, 2020) ascribes a nautical origin to Tom Pepper. This is probably because the earliest occurrences of Tom Pepper that the OED has recorded are:

1-: From The Adventures of Johnny Newcome in the Navy (London: W. Simpkin and R. Marshall, 1818), by Alfred Burton [cf. footnote]:

A Defence which might
Have made Tom Pepper blush outright.

2-: From The Sailor’s Word-Book: An Alphabetical Digest of Nautical Terms, Including Some More Especially Military and Scientific, but Useful to Seamen; As Well as Archaisms of Early Voyagers, Etc. (London: Blackie and Son, 1867), by Admiral William Henry Smyth (1788-1865) and Vice-Admiral Edward Belcher (1799-1877):

Tom Pepper. A term for a liar; he having, according to nautic tradition, been kicked out of the nether regions for indulging in falsehood.

However, nothing in the texts containing the earliest occurrences of Tom Pepper that I have found corroborates the theory advanced by the OED.

These occurrences are, in chronological order:

1-: From Federal Lies, an article about “Craig’s paper, called the British Mercury”, published in the Rhode-Island Republican (Newport, Rhode Island, USA) of Monday 13^th April 1812—the preposition from is missing:

The writers in Craig’s Mercury, for lying, excel Tom Pepper, who was banished [from] h—ll for out-lying Satan.

2-: From a letter to the Editor, by a person signing themself ‘a Friend to Truth’, published in the Republican Star, and General Advertiser (Easton, Maryland, USA) of Tuesday 29^th August 1826—‘a Friend to Truth’ was reacting to a letter by a person signing themself ‘a Citizen’:

If you are a Citizen I know not where you can claim your Citizenship, unless Monchauson may have established a republic of liars—if so, I advise you to emigrate thence as soon as possible, for I am sure you would stand a good chance of being his Secretary of State.—But before you go, I would also advise you to clear your spectacles; for as you are near sighted you might make more mistakes, and tell more falsehoods than you really intended, and thereby excite the jealousy of Monchauson, and get yourself kicked out of the country. You have probably heard that a certain Tom Pepper was kicked out of the lower regions because he out lied his satanic majesty.

3-: From a statement made by a witness at a trial—as transcribed in The Morning Chronicle (London, England) of Saturday 18^th August 1827:

A man is called Tom Truth or Tom Pepper when he is in the habit of telling a falsehood.

4-: From the account of a trial, published in The Leeds Patriot, and Yorkshire Advertiser (Leeds, Yorkshire, England) of Saturday 23^rd January 1830:

A number of witnesses were examined to prove that Burley had given about a thousand and one different versions of the affair since the night in question. He gave a wrong account to the chief constable as to where he slept on that night. In fact, as a liar, “Tom Pepper” is a fool to him!

5-: From the Boston Commercial Gazette (Boston, Massachusetts, USA) of Thursday 21^st October 1830:

Mendacity.—A correspondent of the Centinel of yesterday has perpetrated more falsehoods in three lines, than Honest Old Jack Falstaff did in any three days of his existence. He says the people’s candidate “is the candidate of the Jackson Rulers, the opponents of national roads, canals and railways, the nullifyers [sic] of home trade and domestic manufactures.” Mr. “S.” of the Centinel is evidently a disciple of Tom Pepper, who had the ’cutest tongue at a pinch of any man in Groton.
A National Republican.

6-: From the Emporium and True American (Trenton, New Jersey, USA) of Saturday 9^th April 1831:

The marvel is found in the conduct of the Clay papers, who themselves decline publishing all the toasts, considering them a little too bad. This is out banging Tom Pepper, who, every body knows, out banged the d——l.

7-: From The Sheffield Iris (Sheffield, Yorkshire, England) of Tuesday 5^th July 1836:

If we might be allowed for once to imitate the gentlemanly language of Mr. George Ridge—if we could be permitted to play Tinker to his Sweep, but for a moment, we would not, like him, be continually bandying the terms “wilful and barefaced falsehood,” “implicit falsehood,” &c.: we would at once call him according to his deserts, and apply to him the hacknied [sic] Irish expression, “a mighty big liar.” Tom Pepper was a fool to him—a type of Truth! On Saturday, he out-Ridged Ridge! to the astonishment even of his friends.

Note: The OED erroneously assumes that Alfred Burton was the pseudonym of John Mitford (1782-1831), because this dictionary confuses The Adventures of Johnny Newcome in the Navy (London: W. Simpkin and R. Marshall, 1818), by Alfred Burton, with The Adventures of Johnny Newcome in the Navy, by John Mitford, first published in 1819. The following explanations are from English Coloured Books (New York: G. P. Putnam’s Sons – London: Methuen and Co. – 1906), by Martin Hardie:

In 1818 the Adventures of Johnny Newcome in the Navy, by Alfred Burton, published by W. Simpkin and R. Marshall, has sixteen plates by Rowlandson. This was followed in 1819 by an open imitation by J. Mitford bearing the same title, and illustrated with twenty plates.