Green’s Dictionary of Slang

(US carnival) one who stands in front of a carnival attraction and promotes its virtues to the crowd.

1951		D.P. Mannix Sword-Swallower 62: That gee’s a grindman who thinks he’s a talker. But being a good grind man isn’t as easy as it looks.
1953		F. Brown Madball (2019) 5: Mack Irby stood leaning on a heavy cane listening to the grind of the talker for the unborn show.