From Jemmy, diminutive of James. The verb is from the noun.
Meanings
(Ireland, UK, archaic, slang) A sheep's head used as food.
(Australia, slang) An immigrant.
(obsolete, slang) A greatcoat.
your friend in the green jemmy 1836 March – 1837 October, Charles Dickens, “(please specify the chapter name)”, in The Posthumous Papers of the Pickwick Club, London: Chapman and Hall, […], published 1837, →OCLC:
(alt-of, alternative) Alternative spelling of jimmy (“crowbar”).
I was agreeably surprised by seeing my young friend and companion, Robert Pott, driving up the avenue in a very jemmy equipage. 1808–10, William Hickey, Memoirs of a Georgian Rake, Folio Society 1995, p. 209