Meaning of “sb's heart is in his/her mouth” in the English Dictionary

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"sb's heart is in his/her mouth" in British English

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sb's heart is in his/her mouth

If someone's heart is in their mouth, they are feeling extremely nervous:

My heart was in my mouth when I opened the letter.

(Definition of “sb's heart is in his/her mouth” from the Cambridge Advanced Learner’s Dictionary & Thesaurus © Cambridge University Press)