Dumb originally meant unable to speak, though this older meaning has mostly been replaced by the word mute. You might encounter it in older books or phrases like “struck dumb,” which means suddenly too shocked or amazed to say anything.

Today, people more commonly use dumb to mean stupid or showing poor judgment. When someone says “that was a dumb mistake,” they mean it was careless or silly. A “dumb idea” seems poorly thought out.

This second use creates problems because it confuses not speaking with not thinking, which are completely different things. Many people who cannot speak are brilliant thinkers, scientists, and artists.

When you want to describe something as foolish or ill-considered, you can choose more precise words: foolish, careless, thoughtless, unwise, or ridiculous.