dumb-bell (n.)

also dumbbell, "one of a pair of weighted bars used for exercise," by 1785, earlier (from 1711), according to OED, an apparatus like that used to ring a church bell, but without the bell (hence dumb); used for physical exercise but sometimes also to practice ringing changes. See dumb (adj.) + bell (n.). If this is right, the word must have been transferred; earlier 18c. references make mention of "pulling" or "ringing" dumb-bells and note that it can be done only indoors. The following is a footnote to the 1903 reprint of Joseph Strutt's 1801 "The Sports and Pastimes of the People of England":

The origin of the term is somewhat curious. Dumb-bells take their name by analogy, as was pointed out in Notes and Queries in 1861, "from a machine used for exercise, consisting of a rough, heavy, wooden flywheel with a rope passing through and round a spindle ... and set in motion like a church bell." This statement, however, does not sufficiently explain the transference of such a name to the short bar and rounded lead or iron ends of a hand dumb-bell. This difficulty was explained by the late Chancellor Ferguson in a paper read before the Archaeological Institute in 1895, wherein a dumb-bell apparatus, now at Lord Sackville's seat at Knowle, was described and illustrated. The roller round which the rope winds and unwinds has four iron arms, each of which has a leaden poise or ball at the end, just like the end of an ordinary hand dumb-bell. This Knowle example is fixed in an attic and the rope passed through to a gallery beneath. Anyone pulling the rope would get much the same exercise as in pulling a bell rope in a church tower, but without annoying his neighbours by the noise. There used to be a similar apparatus at New College, Oxford.

Figurative sense of "blockhead, stupid person" attested by 1918, American English college slang.