E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
To show the cloven foot, i.e. to show a knavish intention, a base motive. The allusion is to Satan, represented with the legs and feet of a goat; and, however he might disguise himself, he could never conceal his cloven feet. (See BAG O NAILS, GOAT.)
Real grief little influenced its composition and the cloven foot peeps out in some letters written by him at the period.St. Jamess Magazine.