Green’s Dictionary of Slang

1. a general excl. of disbelief; don’t talk nonsense!

1818		J. Hogg Brownie of Bodsbeck II 27: Get aff wi’ your Hieland impudence – brazen-faced thief!
1904		Marvel 12 Nov. 4: Get off! my articles are dazzling the public.
1942		(con. 1890s) S. O’Casey Pictures in the Hallway 152: Yeh, get off with you! said Dyke viciously.
1953		T.A.G. Hungerford Riverslake 191: Get off yourself. You would have been in it if she would have.
1959		Galton & Simpson ‘Hancock in Hospital’ Hancock’s Half-Hour [Radio script] Oh get off woman.
1967		J. Speight ‘I Can Give it Up Anytime I Like’ Till Death Us Do Part [TV script] Gerroff! Philip as Prime Minister – blimey.
1981		J. Sullivan ‘The Second Time Around’ Only Fools and Horses [TV script] Get off, there’s no calories in weed-killer!
1988		Eble Campus Sl. Mar.
2000		Indep. Rev. 13 Aug. 38: I was like, ‘Get off!’ I honestly didn’t believe him.

2. stop it!

1908		Magnet 20 June 2: Oh, get off!
1943		‘Whitman College Sl.’ in AS XVIII:2 Apr. 154/1: get off me. ‘Stop criticizing me.’.
1953		E. Hunter ‘Vicious Circle’ in Jungle Kids (1967) 31: Hey, man [...] get off that [i.e. indiscreet speech]. Cool it fast.
1960		J. Brosnan Long Season 158: ‘They’ve got the best lobster you ever tasted and you know it,’ she said. ‘You haven’t been to San Francisco yet, dear girl.’ ‘Get off that, will you, Meat,’ she said.
2001		Eble Campus Sl. Fall 4: GET OFF ME – plea to be left alone.

3. (US campus) an expression of admiration.

1988		Eble Campus Sl. Oct.
1996		Eble Sl. and Sociability 56: Many phrases formed around get are fixed expressions that show disbelief [...] or admiration (get back, get off, get up).