Green’s Dictionary of Slang

(US) a compliment, esp. one given in return for a previous compliment paid to oneself; also a compliment passed onto its subject by an intermediary, when the original speaker is too shy to speak themselves.

1925		R. Lardner ‘Zone of Quiet’ in Coll. Short Stories (1941) 70: ‘Oh yes; I almost forgot a T.L. that Miss Halsey said about you. Do you know what a T.L. is?’ ‘Yes.’ ‘Well, then, you give me one and I’ll give you this one.’.
1975		I. Shaw Nightwork 57: ‘While on the subject,’ she said, ‘let me give you a t.l.’ ‘What’s a t.l.?’ ‘[...] T.l. stands for trade last. A compliment. You gave almost the best performance of anyone I’ve slept with in this town.’.