1942, "Marine and Navy usage for any old thing you can't put a name to" ["Life" magazine, July 30, 1945], of unknown origin, perhaps a made-up word. Compare gadget, thingamajig.
Entries linking to gizmo
gadget n.
1886, gadjet (but said by OED corespondents to date from 1850s), sailors' slang word for any small mechanical thing or part of a ship for which they lacked, or forgot, a name; perhaps from French gâchette "catch-piece of a mechanism" (15c.), diminutive of gâche "staple of a lock." OED says derivation from gauge is "improbable."
thingamajig n.
also thingumajig, 1824, see thing. Compare in similar sense kickumbob (1620s), thingum (1670s), thingumbob (1751), thingummy (1796), jigamaree (1824).