When a cow does its business in a field, it often leaves a round, flat cake of dung. That cake is a cowpat. The word is very much at home in British English, on farms and in the countryside. It is not a swear word, but it is a dirty, slightly comic word — children like it, and you would not put it in a serious science paper (there you would say dung). People also joke about stepping in one. Though actually, the “pat” is the same kind of pat as a pat of butter: a small, flat lump.