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serious matter; grave affair; big trouble
「大事」 has another reading with different meanings.
important; serious; crucial
大事だいじにならなくてよかった。
I'm glad it didn't become serious.
小さなちいさなミスが大事だいじになった。
A small mistake became a big deal.
これは大事だいじだから気きをつけて。
This is serious, so be careful.