"Pass me that oojah," you might say. You could mean some useful little device which doesn’t have a name, or something which does have a proper name but which you’ve temporarily forgotten.
The word is rather old-fashioned British English slang. Its heyday was the First World War, when British soldiers created it as part of a private vocabulary. An eyebrows-raised article about the slang of an Army hospital that I found in the Washington Post of 22 July 1917 provides one of its earliest recorded appearances:
"Pass the oojah." says the one-armed man who is playing billiards. What is the oojah? The oojah is any object in Heaven or earth; it is the thing which has no name or the name of which you have temporarily forgotten. The one-armed man, about to make his stroke, requires the little twisted wire bridge, mounted on a lead pedestal, that forms the cue rest which — poor chap! — he ought to have formed with his lost hand. So he demands the oojah, which is army for what-d’ye call-it.
It became greatly elaborated, especially after the war ended and the word was transferred with its speakers to civilian life. It’s known in many forms, including oojah capivvy, oojah-cum-pivvy, ooja-ka-pivi, and oojipoo. Another form is oojah-cum-spiff, which came to mean that something was all right, in order, or OK. This turns up several times in the novels of P G Wodehouse, as here in Right Ho, Jeeves: "Yes, I agree with you, Aunt Dahlia, that things are not looking too oojah-cum-spiff at the moment, but be of good cheer. A Wooster is seldom baffled for more than the nonce."
Woosters may not be, but etymologists often are. Though many of these British Army slang terms of the period were imported from India, oojah has no known origin. If it did come from that country, nobody can tell from what word in which language. (from World Wide Words)