Green’s Dictionary of Slang

L-7 adj.

also l-seven
[L-7 n.]

(US black/teen) unfashionable, unsophisticated.

[US]D. Claerbaut Black Jargon in White America 71: L7 adj., completely out of style; not like the group.
[US]Chicago Trib. 18 Aug. TAB-25/1: Not to mention phrases like ‘L-7 weenie’ and ‘pee-drinking crap-face’.