An exclamation of annoyance and frustration. Normally used when changes of circumstances and factors beyond your control have disrupted plans. Sometimes used in sympathy for someone in the above situation.
Similar usage (but inverse meaning) to w00t! The tone of voice is more important than the word.
"I've just found out I've go to work overtime and can't go clubbing tonight"
"... but now it looks like I can't go - I'd even got a pith helment specially for the occasion."