Green’s Dictionary of Slang

1. used as a challenging phr.

1910		Bulletin (Sydney) 1 Dec. 39/1: ‘Did you go to any of the meetings?’ / ‘Meetin’s, is it? Man, I’ve been goin’ to nothin’ else this last two months!’.
1912		Bulletin (Sydney) 19 Sept. 16/4: ‘Is it dry where you come from?’ / ‘Droy, is it? Whoy, th’ very bir-r-rds thimsilves floy backwards to kape th’ dust out av their eyes!!’.

2. (S.Afr./UK, also izzit?) a non-committal expression used to convey polite disbelief, astonishment, ‘Really?’ ‘You don’t mean to say?’.

1996		CyberBraai Lex. at www.matriots.com 🌐 Isit? This is a great word in conversations. Derived from the two words ‘is’ and ‘it’, it can be used when you have nothing to contribute if someone tells you at the braai: ‘The Russians will succeed in their bid for capitalism once they adopt a work ethic and respect for private ownership’. It is appropriate to respond by saying: ‘Isit?’.
2010		‘A-Z of South African lingo’ in IOL News (Western Cape) 3 June. 🌐 I for Is it/izzit?: An expression of surprise, used even when the preceding statement did not include the verb ‘to be’, eg ‘Brazil first play North Korea’ or ‘I have a toothache.’ Response: ‘Is it?’.
2017		theculturetrip.com ‘Guide to London Slang 10 Jan. 🌐 Is-it (sometimes pronounced iiiiissss iiiiitttt?) – means: really.