Dumb used to mean mute. The phrase meant deaf and unable to speak.

Of course, not being able to communicate leads a lot of people to think someone is stupid, and I imagine that's why dumb is now synonymous with it.

I once met a lady with some severe disabilities, no idea what, in a powered wheelchair at a bar. She couldn't talk, and had a massive keyboard she would sort of flail at until she spelled out the words she was trying to say. It audibly spoke for her.

This lady has two college degrees, writes books, and does art to help promote the concept that disabled people are people too.

Pretty damn impressive. Her and her husband's main gripes were how infantilizing most people are to them. And how expensive good wheelchairs are, lol