endlessly, forever or for an immeasurably long
period of
time. This expression dates back to the
late 1500s, and alludes to cows that,
having been out to pasture, faithfully
return each morning and evening to be milked. The expression till the cows come
home particularly refers to the cows' morning appearance because it implies that some activity (such as partying or revelry) would go on all night until the early morning, literally when the cows would come
home.