Green’s Dictionary of Slang

1. to indulge in sexual foreplay.

1868		H.L. Williams Black-Eyed Beauty 12: I’d on’y like ter know how you warmed up to a fellow so he’d let a watch go so easy as all that!
1904		Lustful Memoirs of a Young and Passionated Girl 35: She went to a dance with her beau one night and got pretty well warmed up. When she went home, about 2 in the morning she asked him in.
1933		J.T. Farrell Gas-House McGinty 79: Now, Mac, is that the way to treat a girlie [male homosexual] that wants to warm up to you.
1953		P. Larkin letter 14 Oct. in Thwaite Sel. Letters (1992) 214: I find old Henry James repulsive sitting there cuddling his ideas, like a butler warming up the undermaids.

2. (US) to pass on information.

1930

E. Glyn Flirt and Flapper 76: Flapper: Lollie warmed him next day on it, and spilled the beans.