A person who gets very angry easily, or a wild and exciting party with lots of people having fun and celebrating.
From 'rage' (intense anger) plus the agent suffix '-er.' The meaning has shifted—it originally meant an angry person, but in recent slang means a party with uncontrolled energy.
Rager is a slang word that captured a generational shift: the same word that once described an out-of-control angry person now describes an out-of-control party—both wild but one's anger and one's joy.
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