I am reliably informed by some of my American friends that there is a difference in meaning, in our respective countries, in the use of the word "ass".
I have used the Oxford English Dictionary meaning which defines the term in the following manner:
(noun) 1 a donkey or similar horse-like animal with long ears and a braying call. 2 (informal) a foolish or stupid person.
Apparently, this is not the common meaning in America where it refers to that part of the anatomy upon which one sits. British people use a different expression.
Thus to refer to a "dumb ass" is not, as in Britain, to refer to a "foolish or stupid person" likened to a dumb donkey but, rather, in America, is cruder and ruder.
I hope this clarifies matters for those who may have misunderstood.
There may be some truth in the old adage that we are two nations divided by a common language.
PS. Moreover, it is not much of an insult in English (e.g. "Oh, you are such an ass, Charles!") since donkeys may be dumb and stubborn but they are still likeable and friendly. Moreover, they were chosen by God who rode into Jerusalem on one. You will notice that donkeys all have the Sign of the Cross upon their backs (see picture). All in all, then, not too bad as insults go...and rather different from the American meaning!