1809, "bell attached to the neck of a cow to indicate her whereabouts" (usually oblong and of a heavy, clanking tone), from cow (n.) + bell (n.). They are cut from sheet metal (mostly copper, or iron coated with bronze) and folded into shape. As a musical instrument (without the clapper) by 1919; in that period associated with, and scorned as an absurdity of, jazz.
Art is long and time is fleeting,
And the man who cannot play,
On a cowbell wildly beating,
Calls it "jazz"--and gets away.
[from "The Psalm of Jazz," Oregon Voter, Aug. 30, 1919]
But an article on cowbells in "Hardware" magazine for Dec. 25, 1897, notes: "Cowbells are made in ten sizes, whose sounds range through an octave. Sometimes musical entertainers who play upon bells of one sort and another come to the manufacturer, and by selection among bells of the various sizes find eight bells that are accurate in scale."