* 바보 (babo): This is the most common and generally understood word for "dumb" or "stupid," but it's quite informal and childish. Think of it like calling someone a "dummy."
* 멍청이 (meongcheongi): This also means "stupid" or "dumb," but it carries a slightly stronger connotation of foolishness or clumsiness. It's still informal.
* 얼간이 (eolgani): Similar to 멍청이 (meongcheongi), this word implies stupidity and lack of intelligence. It's also informal.
* 저능아 (jeoneunga): This word translates to "mentally retarded" or "intellectually disabled." It's extremely offensive and should never be used unless you're discussing a person with an actual intellectual disability in a clinical setting.
Which one should you use? For most casual situations where you want to say someone is a little silly or made a stupid mistake, 바보 (babo) is the safest bet, but remember it's informal. Avoid 멍청이 (meongcheongi) and 얼간이 (eolgani) unless you're very comfortable with the person you're speaking to. Absolutely never use 저능아 (jeoneunga).
If you're talking about someone being unable to speak, you would use a different word entirely, such as 벙어리 (beong-eori), which means "mute" or "deaf-mute."