"whatchamacallit tastes good like a dum-dum cigarette should," Victor put in, in an attempt to settle the subject.
1928, compressed form of phrase "what you may call it." Earliest recorded variant is what-calle-ye-hym, attested from late 15c.
An unspecified or unspecifiable object; something one does not know the name of or does not wish to name; gizmo, thingamajig: lady in a robe and a white whatchamacallit around her head appeared
[1920+; what-d'ye-call-it is attested from 1573, and what-ye-call-'em as London slang in 1710]