脱口而出

tuō kǒu ér chū

to blurt out; to say without thinking

Character by character

脱tuō
slip from

口kǒu
mouth

而ér
and

出chū
out

他没多想，答案就脱口而出了。

Without a second thought, the answer just came out.

Tone: neutral.

Good (a quick answer) or bad (an unguarded remark); context decides.

Learn it in context

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