Brewer's: Heart in his Mouth
His heart was in his mouth. That choky feeling in the throat which arises from fear, conscious guilt, shyness, etc.
“The young lover tried to look at his ease, ... but his heart was in his mouth,” —Miss Thackeray; Mrs. Dymond, p. 156.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894