Heart in his Boots
His heart fell into his hose or sank into his boots. In Latin, “Cor illi in genua decidit. ” In French, “Avoir la peur au ventre. ” The two last phrases are very expressive: Fear makes the knees shake, and it gives one a stomach-ache; but the English phrase, if it means anything, must mean that it induces the person to run away.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894