Answer: Asinine

In modern use the term “asinine” is used to indicate that a person is failing to exercise sound judgement, not using their intellect, or behaving in a way that is ridiculously below the rationality and common sense one would expect from even a person of average ability.

We owe a debt of gratitude to Latin for giving us such a compact and rolls-off-the-tongue word for such a lengthy judgement of another person and then, in turn, we should probably give the lowly donkey a nod of acknowledgement too.

For you see, the modern English word asinine is derived from the Latin word asinīnus (“of a donkey or ass”) and in more archaic English use literally meant “pertaining to the characteristics of donkeys” but over time came to mean “You sir, are acting like quite the ass” and has became more metaphorical in application than literal.