I first heard this word in 1991. My friend Katie said it in seventh grade. I believe it is a euphemism for the s word (you know, 'poop') - I don't know if I'm allowed to print it here. Anyway, I think it's cute and have only heard it once since - from a co-worker named Christina who thought she made it up.
Upon realizing you don't have enough change for pop/upon realizing you set off your car alarm... "Oh poopydust!" Yeah, it's usually said alone. Giggling usually ensues.