To do in is to kill somebody — or to wear them out completely. Both are informal, and one is a great deal more serious.
The killing sense belongs to crime fiction and to jokes rather than to real reporting, so threatening to do somebody in is almost never literal. The exhausting sense is common and harmless, and being done in after a long day is what everybody says. The adjective done in means shattered. Context and tone separate the two entirely.
Practised in A Man Called Ove, A Song of Ice and Fire
First appears in chapter 11 of A Man Called Ove.
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