L-7, n. — Green’s Dictionary of Slang

L-7 n.

also l-seven

[the L and the 7 when put together form a SE square thus a pun on square n. (4b); the word can be accompanied by using thumb and forefinger extended at right angles, forming an L and a 7, and when the two are combined they form a square]

(US black/teen) a conventional, tedious person, unsympathetic to teen interests.

1956		Artie Shaw in West Coast Jazz 79: Hollywood’s latest lingo for a square: for an L and a 7 with your fingers and that’s what you get [W&F].
1966		Baker Aus. Lang. (2nd edn).
1972		D. Claerbaut Black Jargon in White America 71: L7 n. a square; unpopular person; one who is not like the group.
1996		Eble Sl. and Sociability 21: The term L7 for ‘a socially inept person, a square’ should be accompanied by the thumb and index finger of one hand (shaped like an L) joining the index finger and thumb of the other (shaped like a 7) to form a square.
1997		L. Pettiway Workin’ It 112: I was becoming a L7, you know, and I stepped off from there. Went on back home to Mom.