what's 'It's screwed' means?
I know 'screwed' means 'fastened by a screw or screws',but what's 'it's screwed' means? and how can I use it?

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"Screw" is used in the US to mean "f**k", in a PG-rating way.  Many of the colloquial uses of f**k apply to "screw".

"It's screwed" can mean something doesn't function, or it's damaged beyond repair.

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It can mean a number of things. The one you mentioned is the literal use. But the "it's screwed" can mean "its doomed to fail". One could say, "i didnt study for this test, I'm screwed" as in "unlikely to succeed.

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Its Slang it means its doomed to failure or won't work :)