until the cows come home

Etymology:

• Possibly from the fact that cattle let out to pasture may be only expected to return for milking the next morning; thus, for example, a party that goes on “until the cows come home” is a very long one.
• Alternatively, the phrase may have a Scottish origin, and may derive from the fact that cattle in the Highlands are put out to graze on the common where grass is plentiful. They stay out for months before scarcity of food causes them to find their way home in the autumn for feeding.