The English word “dumb” didn't develop its stupid sense until the 19th century, according to Ayto, “presumably under the influence of” the German dumm and the Dutch dom, adjectives meaning stupid. The OED has two 18th-century citations by the English Roman Catholic priest Alban Butler for “dumb” meaning stupid.

When did the word dumb start?

The word dumb itself is quite old, with its first recorded use dating back to before the year 1000. It's a cognate with the Old Norse dumbr, Gothic dumbs, Old Saxon dumb, Old High German tump, and German dumm.

Where did the word dumb come from?

Dumb is the Old English word that means "mute, speechless," and itself came from an even older word dheubh meaning "confusion, stupefaction, dizziness." Today, dumb still means "unable to speak," but it doesn't have anything to do with intelligence.