from The American Heritage® Dictionary of the English Language, 4th Edition
- interj. Used to express acknowledgement of a minor accident, blunder, or mistake.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- interj. Acknowledging a minor mistake.
- interj. Used sarcastically to acknowledge a major mistake.
A presumably 'natural' exclamation, only attested since 1933, possibly altered to (or from) whoops (attested since 1933) (Wiktionary)