Green’s Dictionary of Slang

do you understand? esp. referring to the point of a joke.

1937		R. Westerby Wide Boys Never Work (1938) 161: And you – Mick – you’re through! Get it?
1952		(con. 1920s) ‘Harry Grey’ Hoods (1953) 92: Pipy pulls a switcheroo on Nutchy, get it?
1960		H. Pinter Caretaker Act III: I’m staying on here as caretaker! Get it! Your brother, he’s told me, see.
1976		P. Theroux Family Arsenal 33: I deal on the continent. Arab hardware. Get it?
1983		S. May No Exceptions in Best Radio Plays (1984) 119: See these lines [...] across the page. They’re for writing on. Like railway lines, and your pen’s the train. Get it?
1992		K. Lette Llama Parlour 172: I wancha to know, right [...] that I ain’t no butt-head, got it?
1999		Observer Escape 11 July 9: Spain: ‘The perfect recipe for a holiday’. Geddit?
2000		Guardian Guide 22–28 Jan. 28: Do you get it?