Origin: 1940–45; sp. of what you may call it reflecting affrication of t before y and reduction of vowels; cf. similar metric pattern and internal əmə in rigamarole, thingamagig
n. a name for a person or thing whose real name has been forgotten or is being avoided. : I lost my—you know—my whatchamacallit—my watch!
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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Word Origin & History
whatchamacallit
1928, compressed form of phrase "what you may call it." Earliest recorded variant is what-calle-ye-hym, attested from c.1473.