My mother used to say this before she went out on Fri nights. I had thought that she made it up but then figured it came from some nursery rhyme that she'd heard growing up, She'd say,

I'm going downtown to smoke my pipe
And I won't be back 'til Saturday night
And if you let that old witch in
I'll beat you 'til your black and blue

Us three kids would laugh and she'd be out the door before we realized she'd left us with the babyysitter. :^)

Now I know it came from a children's game... Thanks