Green’s Dictionary of Slang

a term used when one cannot find the correct description for an object or person.

1917		W. Muir Observations of Orderly 229: A few other slang words which I have come across in the hospital, and which seem to me to bear the mark of the old army as distinct from the new are: [...] ‘oojah,’ anything (similar to thingummy or what-d’ye-call-it).
1919		W.H. Downing Digger Dialects 28: hoojah — What’s-his-name.
1921		N&Q 12 Ser. IX 466: Oojiboo [...] Signified anything.
1921		(con. WWI) N&Q 12 Ser. IX 418: ‘Oojarkapiv’, of which only the first two syllables were commonly used, I always heard aspirated.
1924		(con. WWI) A.G. Pretty Gloss. Sl. [...] in the A.I.F. 1921–1924 (rev. t/s) n.p.: oojah. Any article; one of the names given to a Fullerphone, when in the forward area when it was devised to keep its presence unknown to the enemy.
1925		(con. WWI) Fraser & Gibbons Soldier and Sailor Words 215: Oojiboo: Much the same as Oojah. [...] Oojah (also Ooja-Ka-Pivi): A substitute expression for anything the name of which a speaker cannot momentarily think of.
1930		(con. 1914–18) Brophy & Partridge Songs and Sl. of the British Soldier 145: Ooja.—Or, in full, ooja-cum-pivvy. A military equivalent for ‘thingamytite’.
1941		in AS XVI:3 Oct. 239/2: Oojah, a synonym of gadget.
c.1943		‘Bill O. Lading’ You Chirped a Chinful!! n.p.: Oojah: Gadget.
1971		(con. WWII) B. Aldiss Soldier Erect 94: I’ve seen blokes in hot countries go clean round the oojar because of the perverted practices of native women.
1982		N. Keesing Lily on the Dustbin 66: Mythical personalities or nonsense phrases for people and things whose names are forgotten or elusive are very common: ‘Whose-me-whatsit’, ‘thingmejig’, ‘oojar-capivio’ or ‘whatchamacallit’ [...] ‘whoflickie’, ‘whasmigig’.