This is a slang word used all the time in spoken English.  Literally, it refers to someone with a mental handicap or problem from birth.  In daily use, people don’t mean it this way.  We use it to say that someone or something is stupid.  You should be careful with this word because you can offend some people with it.

Here are some common ways you might here it in movies or real life:

He’s such a retard.  He doesn’t even know how to use a cell phone.

I saw that movie.  I seriously think you’ll hate it.  It’s pretty retarded.

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