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When you simply cannot remember the name of something, what do you do? Exactly. You make up a nonsense word to substitute for the forgotten word. The same applies for the names of gizmos, usually electronc or mechanical, whose name we don't know.
Curiously, most European languages have some equivalent. In French, it is 'bidule' or 'machin'. In Dutch, it is 'Dingen'. In Italian, it is 'aggeggio'. All of these European equivalents will have their local meanings which shade the use specifically towards machine type devices or just stuff in general.
Thingamabob was used as early as 1750. Back then folks just said 'thingumbob' or 'thingananny' or some 'thingajig'. The third syllable---which may be 'a', 'e', 'u', or 'um'---was added only after the word was in use for some time.
Other such nonsense words were: doohickey, usually referring to a rather small thingamabob; doodad, whichamacallit, whatamacallit, or, just thingy.
One other variation of thingamabob came to light in my voluminous research. Edgar Allen Poe wrote a curious, sometimes comical, relatively unknown story entitled 'Literary Life Of Thingum Bob, Esquire'. If you are interested in mid-19th century humor, have a go. Otherwise, give it a pass.
Thingum Bob, Esq