Definition from macmillandictionary.com
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This is a potentially deadly disease.
Too many water changes can prove deadly to these delicate fish.
The police charged him with possession of a deadly weapon.
A deadly silence followed her announcement.
Don’t say anything to anyone – this is meant to be a deadly secret.
It started as a joke, but soon they were fighting in deadly earnest.
Some of our lectures were pretty deadly!
The Australian captain is a deadly goal scorer.
That teacher is deadly! That was a deadly class.