Durham man, n. — Green’s Dictionary of Slang

Durham man n.

[proper name Durham, home of high-quality mustard, which was ground between two stones]

one whose knees knock or rub together.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Durham Man. Knocker kneed, he grinds mustard with his knees: Durham is famous for its mustard.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn).
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.