E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Heart in his Boots.
His heart fell into his hose or sank into his boots. In Latin, Cor illi in genua decdit. 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.