Look it up in the dictionary. The original and traditional meaning of "dumb" is "mute": physically unable to speak.

So you mean "mute" when you call some one "dumb"? (Elsewhere, I've said, "Thou shalt not attribute!". That is, attribute an inherent quality to a person. Don't say to a child or older person, "You're bad!" Say, "You're behavng badly!", or equivalently the gerundish, "You're badding!") When you say, about a book or movie or party or whatever, "This is dumb!"", do you mean "It's mute!"?

This is not only foolish or ignorant behavior, but it is INCONSIDERATE OF THOSE WHO ARE MUTE! And I've heard many of them indicate that they recognize the slight -- intended or not.

The thoughtful ethical person corrects and avoids such ethical-leaks.