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That phrase dates to at least 1825, and has been attributed to the Scottish, but according to the Urban Dictionary, there are two definitions  for “Until the Cows Come Home:”

1) For a very long time.
2) From now until whatever time the fat broads you live with return to the house.

Personally, I like the second definition, but I’ll deny that admission because in my case it doesn’t apply, especially not on The Overnight Open Thread.