According to the OED the term has different spellings and its origin remains obscure.
Oojah, oojahslang (Also oojar, ujah) of uncertain origin.
A substitute expression used to indicate vaguely a thing of which the speaker cannot at the moment recall the name, or which he does not care to specify precisely; a ‘what-you-may-call it’, gadget.
1917 W. Muir Observations of Orderly xiv. 229 ‘Oojah’, anything. > - 1925 Fraser & Gibbons Soldier & Sailor Words 215 Oojah (also Ooja-ka-pivi), a substitute expression for anything the name of which a speaker cannot momentarily think of, e.g. ‘Pass me that h-m, h-m, oojah-ka-pivi, will you?’
1930 Wodehouse Very Good, Jeeves! i. 25 ‘All you have to do,’ I said, ‘is to carry on here for a few weeks more, and everything will be oojah-cum-spiff.
(OED)
According to the following source the most used form is oojah.
oojah or oojiboo or oojamaflip:
- noun informal used when one cannot think of or does not wish to use the name of something.
ORIGIN early 20th cent.: of unknown origin.
Ngram: oojah.
