barbell n.

late 12c., "stake or rod of iron used to fasten a door or gate," from Old French barre "beam, bar, gate, barrier" (12c.), from Vulgar Latin *barra "bar, barrier," which some suggest is from Gaulish *barros "the bushy end" [Gamillscheg, etc.], but OED regards this as "discredited" because it "in no way suits the sense." Welsh bar "a bar, rail," Irish barra "a bar, spike" are said to be from English; German Barre, Danish barre, Russian barŭ are from Medieval Latin or Romanic. 

The general sense of "anything which obstructs, hinders, or impedes" is from 1530s. Of soap, by 1833; of candy, by 1906 (the process itself dates to the 1840s), both from resemblance of shape. The meaning "bank of sand across a harbor or river mouth" is from 1580s, probably so called because it was an obstruction to navigation.

Bar graph is attested from 1925. Bar code first recorded 1963. Behind bars "in prison" is attested by 1934, American English.

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.