Old English þymel "sheath or covering for the thumb," from thuma (see thumb (n.)) + instrumental suffix -el (1), used in forming names of tools (compare handle (n.)). The unetymological -b- appears mid-15c. (compare humble, nimble, etc.). 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.)).