Green’s Dictionary of Slang

1. (US) a fool, an idiot.

1958		W. Talsman Gaudy Image (1966) 172: This guy is one of those gobby guys [...] fat like that jerkwater at the club.
1992		Bentley & Corbett Prison Sl. 35: Jerkwater One who is slow witted. (Archaic: pop corn).

2. see jerkwater town under jerkwater adj.