During World War I British soldiers invented a number of new slang words, from blighty to napoo to toot sweet. Unlike most of these, oojah is not a mangling of a foreign term, appearing to have been created sui generis. It is a very useful word, as it can refer to anything... it is simply a shorter, more efficient way of referring to a doohickey, thingumabob, or whatchamacallit.
It took well to civilian life, and was used and elaborated on with great whatsit. Mutant forms include oojah capivi, oojah-cum-pivvy, oojah-ma-flip, and oojiboo (oft incorrectly cited as oojipoo. Don't make this mistake!). The Canadians troops, being right oddballs, used hoozamekloo, which may or may not be related.
An adjectival variant, oojah-cum-spiff, was introduced by Bertie Wooster to mean 'a-ok'. This coinage has gained some popularity, although this form hardly seems in the spirit of a metasyntactic variable.
Brevity Quest 2016