a thing whose name one forgets, does not know, or prefers not to mention
Since I don't have access to the OED, can someone tell me its etymology and whether it's commonly used nowadays?
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The term 'oojah' is soldiers' slang, and was known as a 'gadget' during WWII according to the Dictionary of service slang, compiled by Park Kendall (1944)
A theory on the origin of ooja-ka-piv:
"Widget" is a bit more recent than "gadget," means roughly the same thing, and may be simply an alteration of the word "gadget." One theory, in fact, holds that "widget" arose in the Royal Navy as a contraction of "wifflow-gadget," also known as a "hook-me-dingy" or "ooja-ka-piv." All those terms, like "gadget" itself, were invented by sailors who had momentarily forgotten what to call a particular piece of equipment.
& the term "Oojieoo" -notably a song title found in a 1910 Catalog of copyright entries
Oojieoo. The hardest worked word. Signified anything, its nearest equivalent being "gadget." Once I heard this dialogue :
—Lorry driver: "Sorry, Flight, but in the fog a fellow passed me on the wrong side and carried away my oojiboo."
Flight Sergeant: "That's the second side-lamp you've done in this week." -Origins and meanings of popular phrases & names, including those which came into use during the great war, by Basil Harg ave. 
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.
According to the following source the most used form is oojah.
- 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.