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
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Examples from the Web for thingamajig
Historical Examples of thingamajig
By Jove, Madge, I jolly well hoisted him with his own thingamajig!The Prodigal Father
J. Storer Clouston
also thingumajig, 1824, see thing.
Online Etymology Dictionary, © 2010 Douglas Harper