Green’s Dictionary of Slang

1. extraordinarily good.

1917		B. Lee [perf. ] ‘Daisy Dormer’ 🎵 Old Farmer Brown from Lancasheer says / ‘Eh, by gum, it’s extra, and the wife’s not here’.
1945		A. Kober Parm Me 9: Eh, she ain’t so extra [...] She got no shape, she got no good looks.
1960		‘Nino Culotta’ Cop This Lot 159: ‘How did you go, Joe?’ ‘Extra, matey. Easy.’.
1965		J. O’Grady Aussie Eng. (1966) 38: Our beer is ‘extra grouse’.
2017		‘10 Millenial Words’ in femalenetwork.com 23 May 🌐 Extra Definition: adj. Something that is over the top, excessive, overly dramatic. How to use it: ‘Oh my goodness, she actually made a cake for her dog! That's so extra!’.

2. (N.Z.) as an intensifier.

1988		McGill Dict. of Kiwi Sl. 43/1: extra enhancing intensive which can reverse the meaning of the parent word.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].

3. (US campus) extreme; unrestrained.

2012		C. Eble (ed.) UNC-CH Campus Sl. 2011 4: EXTRA — too much, excessive: ‘That girl just broke into song in the grocery store. She’s so extra.’.
2017		theculturetrip.com ‘Guide to London Slang 10 Jan. 🌐 Extra – OTT basically.
2018		Twitter 16 Feb. 🌐 Can we all just enjoy the movie without being extra?