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English definition of “lighten up”

lighten up

verb [T] phrasal verb with lighten  /ˈlɑɪ·tən/ us  

(MAKE NOT SERIOUS)

stop being so serious: Why don’t you just relax and enjoy yourself? Lighten up!
(Definition of lighten up from the Cambridge Academic Content Dictionary © Cambridge University Press)
What is the pronunciation of lighten up?
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