Yours, Etc.: Origins and Uses of 8 Sign-Offs
Old English fyst, from West Germanic *fustiz (cf. Old Saxon and Old High German fust, Old Frisian fest, Middle Dutch vuust, Dutch vuist, German Faust), from Proto-Germanic *fukhstiz, probably ultimately from PIE *penkwe- "five" (cf. Old Church Slavonic pesti, Russian piasti "fist").