Green’s Dictionary of Slang

1. (also yells) something or someone hilarious.

1926		E. Hemingway Sun also Rises 179: ‘Bill’s a yell of laughter,’ Mike said.
1938		in N. Marsh Artists in Crime 238: ‘Well, of course!’ exclaimed Miss O’Dawne, greatly diverted. ‘Aren’t you a yell!’.
1949		E. Coxhead Wind in West 32: All these doctors and their ecologists – what a yell [OED].
1957		M. Shulman Rally Round the Flag, Boys! (1959) 52: ‘I’m going to town meeting now.’ ‘That ought to be yells.’.

2. (US Und.) the betrayal of one’s confederates.

1950

Goldin et al. DAUL 242/1: Yell, n. The act of yelling.

3. (US black) someone considered excellent, attractive.

1953

L. Durst Jives of Dr. Hepcat (1989) 9: Little one you are a triple scream, and one big yell. Yes you must have it much made, because you don’t rattle when you roll.