big L, n. — Green’s Dictionary of Slang

big L n.

[abbr. on pattern of big C n.]

1. (US) love.

1982		Eble Campus Sl. Spring 5: big L, little L – big L constitutes love, whereas the little L is only lust.
1987		Night Court [NBC-TV] n.p.: You don’t suppose it could be the Big L? [HDAS].

2. (US campus) a person who is a failure, a loser n. (1)

1984

Eble Campus Sl. Sept. 1: the big L – loser: He’s the big L.