Multiple definitions include:
1. An exclamation of mild annoyance towards a nearby individual.
2. Doing something quickly, rushing, so to avoid certain undesirable situations.
1. "I regret to inform you that your mother is dead."
"ahh.. you bloody bumwunce!"
"dont shoot the messenger, sir."
2. "if you dont finish your article, you'll be in trouble!"
"ah, give me a second, i'll bumwunce it."