Green’s Dictionary of Slang

an unnamed object or person; often used euph.

1705–07		N. Ward Hudibras Redivivus II:5 24: You taudry Fop, with Diamond Ring; / You little Thingum of a Thing.
1707		N. Ward London Terraefilius I 12: That Effeminate Skeleton of a Beau, that Pissle-wasted Thingum of a Prodigal.
1716		R. Neve Merry Companion 165: I should have gone to Good Man What-d’ye-call-’ems, and I am got to Good Man Thingaments.
1721		‘The Sick Wife’ in Pleasures of Coition iv: I had the finest Thingum for ye.
1793		Sporting Mag. May II 127/2: Your two-inch waist, and all your bunch of thingums!
1906		J.W. Carr ‘Words from Northwest Arkansas’ in DN III:ii 161: thingumadoodle, n. What-do-you-call-it? ‘Reach me that thingamadoodle there.’.
1953		Berrey & Van den Bark Amer. Thes. Sl. (2nd edn).
1984		D. Evans et al. Serious Bizness 9: You mean the little thingumadoodle?
1995		E. Clifford Flatfoot Fox 26: ‘What’s a thingumadoodle?’ asked Secretary Bird.