Fannie Mae-No Politics (FNMA)

[Daveknowsnothing]

One down...one to go!!

Possibly from the fact that cows may be expected to return for milking in the morning; thus, for example, a party that goes on "until the cows come home" is a very long one.
Alternatively, assuming a Scottish origin, from the fact that cows in the highlands are put out on the common grazing until the end of summer, when they will find their way back home to be let in for feeding. Grass on the hill (common) is plentiful, so cows may stay out for months before scarcity of food brings them home in the autumn.