Ass

In the sense of a stupid, ignorant person dates from the late 16th century, from the allusion to an ass or donkey being regarded as a rather dull, stupid animal. In America, ass (the equivalent of the British word arse) meaning the buttocks or rectum dates from the mid-19th century. There are a great many expressions in both British and American English that use the words ‘arse’ or ‘ass’ and it would be tedious to include them all. In such expressions, ‘arse’ and ‘ass’ are mostly interchangeable, but not always. For example, in British English ‘a silly arse’ is a mild expletive to describe a silly or foolish person, whereas in American English, ‘a silly ass’ is more likely to describe a person as a silly ass, as in donkey rather than a posterior.