Its “the jig is up.” Based on the use of a “jig” to keep a saw or other cutting fence in place. When the jig would come up, the cutting edge would go all over the place; the pieces being cut would fly all over the place.
A “gig” is a contract job. The first uses I could see were from musicians. But I used to call my photo jobs gigs.
The G in “gig” is like the sound used in “go.” The sound in “Jig” is like “jar.”
That is my grammar police and colloquialism lesson for the day.
What is a jig anyway?
Sorry posted too soon. I meant to say I never knew what a jig was anyway. Thanks for the info!