Why do English people over 40 think dumb means cannot speak in old English, when Americans say it means stupid?

The word "dumb" originally meant "incapable of speaking, mute." Often the term was applied to deaf people, who since they could not hear sounds, did not learn how to speak. Hence the old term "deaf and dumb."

In traditional biblical translations, a mute person is referred to as dumb. Isaiah prophesizes "the dumb shall speak". At least, that is how the King James and other translators put it into English, reflecting how the word was used in English at the time.

People with such an affliction made noises which were easy to misinterpret as a sign of stupidity, and thus the word , unfortunately and unfairly, came to mean "stupid." But that is a relatively modern development.

It's not "old English".

"Dumb" does mean "unable to speak".

Using "dumb" to mean "stupid" is American slang, but I suppose it might have come from "struck dumb", which means being so startled or shocked as to be rendered speechless for a few moments.

It means both things. Americans stopped using the term because it sounded offensive to those unable to speak.

It had both meanings. Many words have dual meanings depending on the context it’s used.

It means both

Old English? That's a rather different language not much like Modern English. No one has spoken Old English as their native tongue for getting on for a thousand years.

:-)

Apparently English people over 40 who believe this have been confined in their rooms at Oxford for far too long...🤣🤣🤣

It has both meanings.

Even in American English it had the meaning of mute as well as stupid or foolish. In Rochester we had The School for the Deaf and Dumb that closed when I was young and replaced with something less antiquated