Englishfor English speakers
deadly
Adjective
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Something that is deadly can kill a person or thing.
The poison is very deadly.
serious
Adjective
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A problem or situation is serious if it is not funny or comical, but important.
This is a serious problem. We must do something.
—
A person is serious if the person is not laughing or joking, but saying what they mean.
I see. When you said the room was empty, you were serious!
The woman's face looked serious, so I knew she was saying something important.