E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A law discovered by Jacob L. Grimm, the German philologist, to show how the mute consonants interchange as corresponding words occur in different branches of the Aryan family of languages. Thus, what is p in Greek, Latin, or Sanskrit becomes f in Gothic, and b or f in the Old High German; what is t in Greek, Latin, or Sanskrit becomes th in Gothic, and d in Old High German; etc. Thus changing p into f, and t into th, pater becomes father.