Green’s Dictionary of Slang

1. an idiot, a fool.

1858		in Thoreau Autumn (1892) 94: I see dumb-bells in the minister’s study and some of their dumbness gets into his sermons.
1860		‘Q.K. Philander Doesticks’ Lady of the Lake 24: Put up the sword – I wish I could as well into the skies, put up this large dumb bell. (Pointing to Blanche, dead on the floor.).
1890		Nat. Police Gaz. (NY) 4 Jan. 11/1: [headline] That Big Dumbbell [...] It is an open question [...] who is the strongest man in the world.
1918		Hall & Niles One Man’s War (1929) 307: Poor dumb-bell!
1922		Maui News (Wailuku, HI) 15 Dec. 5/4: Only a dumb-bell calls another person a dumb-bell.
1929		L. Thomas Woodfill of the Regulars 31: If there was a bigger dumb-bell in the outfit than I was I didn’t see him.
1931		Ade Old-Time Saloon 20: The distillers, the brewers and the retail dealers in wine, liquors and cigars were a lot of overbearing and impudent dumb-bells.
1945		A. Kober Parm Me 61: Guess you think I’m an awful dumbbell or something.
1954		Wodehouse Jeeves and the Feudal Spirit 65: Well, speak up, dumb-bell.
1955		B. Appel Tough Guy [ebook] [W]hat a dumbbell! [...] full to the ears with all the junk they taught in the schools.
1967		L. Bruce Essential Lenny Bruce 64: Maybe ah am a dumbbell, maybe that’s it.
1971		B. Moyers Listening to America 138: The average Kansan is not a dumbbell.
1984		LaBarge & Holt Sweetwater Gunslinger 201 (1990) 114: The first hotel I can find, dumbbell.
1989		H. Rawson Dict. of Invective (1991) 130: Dumb makes an excellent prefix or intensifier. For example: dumb-ass, dumbbell, dumb blonde, dumb bunny, dumb cluck, dumb Dora, dumb-dumb.
1999		S. King Hearts in Atlantis (2000) 285: Kicking people when you’re wearing sneakers is for dumbbells.

2. see bell n.¹ (7)