Old English þymel "sheath or covering for the thumb," from thuma (see thumb, n.) + -el(1), used in forming names of instruments (compare handle). Excrescent -b- began mid-15c. (compare humble, nimble). Originally of leather, metal ones came into use 17c. Related: Thimbleful. Thimblerig, con game played with three thimbles and a pea or button, is attested from 1825 by this name, though references to thimble cheats, probably the same swindle, date back to 1716 (see rig, v.).