I think it's usually "oojah-kapivvy", and means "thingamajig" or "whatsitsname". It almost certainly comes from the slang of the British army in India, and may originally have had some kind of sexual connotation in whatever local dialect phrase it was corrupted from (if that makes any sense!).
I'm surprised Squarebear hasn't heard of it (sorry, Squarebear - no offence) as in my experience it, or a variant on it, is a fairly common expression. Maybe it depends which part of the country you're in - though if it's used in NE Scotland ,here in the West Midlands and in the Home Counties (a friend of mine from Middlesex is familiar with it) it must be pretty widespread.