Green’s Dictionary of Slang

(US) any object that one has forgotten the name for.

[1918		‘Bartimeus’ Navy Eternal 116: ‘What I should like to know is, what the deuce is a doo-hickie?’ [...] ‘Well, [...] you know more or less what a gadget’s like?’ ‘Yes.’ ‘And a gilguy?’ ‘Yes.’ ‘ Well, a doo-hickie is something like that, only smaller as a rule.’].
1934		Weseen Dict. Amer. Sl. 338: Gadget ... Gilgadget is a variant.
1944		P. Kendall Dict. Service Sl. n.p.: gilgadget . . . a gilhickey or a gimmick or a gadget.
1954		in PADS [DARE].