Is it ensure, insure, or assure?
"weighted bar used for exercise," 1711, originally an apparatus like that used to ring a church bell, but without the bell (hence dumb); used for exercise but sometimes also to practice ringing changes. Figurative sense of "blockhead, stupid person" attested by 1918, American English college slang.
A stupid person; idiot: She's an awful dumb-bell
[1918+; fr dumbbell, a kind of weightlifter's barbell, attested from the 1880s]