Green’s Dictionary of Slang

1. the truth, the right place, the ideal situation, opinion, experience, an expression of approval/affirmation.

1854		G.J. Whyte-Melville General Bounce (1891) 132: We might get money – ay, plenty of it – if you were only like the rest: you’re too mealy-mouthed, Mrs. Blacke, that’s where it is.
1896		P.L. Dunbar ‘When De Co’n Pone’s Hot’ in Lyrics of Lowly Life 133: When the chittlins is a-splutter’n / So’s to show you whah dey’s at.
1929		Coot Grant & Socks Wilson ‘Can I Get Some of That?’ 🎵 All I want to know just where it’s at. / I’m a hungry cat.
1968		N. Heard Howard Street 73: You’re still young yet. You really don’t know where it’s at.
1972		(con. c.1930) G. Sothern Georgia 231: ‘This [burlesque] is it, this is what’s happening, this is where it is for you’.
1978		R. De Christoforo Grease 157: Be yourselves and have a ball, that’s where it’s at.
1988		Ice-T ‘Power’ 🎵 Loved by troopers who know where it’s at.
1999		Indep. Rev. 4 Aug. 9: For those who know where it’s at.
2001		K. Waterhouse Soho 130: Soho was supposed to be where the action was, where it was all at as the old phrase had it.

2. (also where are you at?) as a greeting.

1970		J. Conaway Big Easy 14: Hey ya muthah, wheah ya at?
1996		Eble Sl. and Sociability 100: Question forms such as these versions of how are you? signal that a response is in order: [...] what’s up?’s up?what’s up, G?what you know?where it’s at? and qué pasa?