Reaney & Wilson (A Dictionary of English Surnames) has a ...rather long header starting with Folk: Folk, Folke, Folkes, Folks, ffoulkes, Foulkes, Foulks, Fulk, Fulkes, Fulks, Fewkes, Foakes, Focke, Fokes, Fookes, Fooks, Foukx, Foux, Fowke, Fowkes, Fuke, Voak, Vokes, Volk, Volke, Volkes, Volks. They all go back to Old French Fulco, Fouques or Old German Fulco, Folco, personal names that ultimately derive from a word meaning "people".