a gadget or other thing for which the speaker does not know or has forgotten the name.
Origin of thingamajig
1870-75;thingum or thingummy + jig1, metrically patterned like gobbledygook, rigamarole; thingum perhaps orig. from the gradational compound thingum-thangum (based on thing1) with the same terminal syllable as in crinkum-crankum, harum-scarum, trinkum-trankum, the ultimately orig. of which is unclear