the jig is up

Etymology
is an old term for a lively dance, and in the the word also became slang for a practical joke or a trick. This idiom derives from this obsolete slang word.

Phrase

 * 1)  An expression used to mean "We have been caught out and have no defense", or if spoken to a person who has just been found out as the perpetrator of an offense, where it means "You've been discovered".