(simple past the jig was up)
- (US, idiomatic, dated) An expression used to mean "We have been caught out and have no defense", or if spoken to a person who's just been found out as the perpetrator of an offense, it means "You've been discovered."
'Jig' is a very old term for a lively dance, and in Elizabethan times the word also became slang for a practical joke or a trick. 'The jig is up' - meaning your trick or game is finished, has been exposed, we're onto you now - derives from this obsolete slang word. From "Encyclopedia of Word and Phrase Origins" by Robert Hendrickson (Facts on File, New York, 1997)
Another more speculative origination was posted prior to the referenced one above. It suggests the phrase may refer to a common fishing situation where a fisherman boasts they have hooked on to a large fish, at which point an unconvinced onlooker quickly refutes this claim because they can see that the fisherman's "jig is up." A jig is a common form of fishing tackle, typically arrangements of hooks, sinkers, metal bracing, plastic beads and swivels designed to target specific fish species.