Ass is an informal English name for a Donkey. In England, a Donkey is perceived as a foolish animal.
Thus, when someone/something is referred to as an Ass, it simply means that the person/thing is stupid.
In Oliver Twist, 1838, according to Charles Dickens, Mr. Bumble whose wife is domineering was unhappy when he was told in court that "...the law supposes that your wife acts under your direction".