Green’s Dictionary of Slang

stupid, foolish.

1922		Colton & Randolph Rain II 141: I guess I’m the dumbbell king, all right.
1927		H.C. Witwer Classics in Sl. 85: He’s got to take off a lot of weight on the account of makin’ that dumb-bell agreement with Shylock.
1936		M. Levin Old Bunch (1946) 49: Why waste himself on a dumbbell crowd like this?
1943		T. Thursday ‘Romeo’s Juliet’ in Sports Fiction Fall 🌐 Mamma never reared no dumbbell brats.
1967		K. Kolb Getting Straight 53: She remained after class [...] shyly apologizing for her presence in what she forthrightly called ‘dumbbell English’.
1975		L. Bangs in Psychotic Reactions (1988) 170: A panderer living off the dumbbell nihilism of a seventies generation that doesn’t have the energy to commit suicide.