Green’s Dictionary of Slang

(US) anyone who has a smooth, seductive line of talk.

1969		Current Sl. III:4 9: Snowman, n. A fellow with a ‘line’ or with a practiced script to impress a girl.
1979		B. Gutcheon New Girls (1982) 198: I knew he was supposed to be a snowman, but I didn’t know he was a cruel shit.