Rabid comes from Latin rabidus ('raging, mad, furious'), from rabere ('to rave, be mad'), the same root that gives rabies. The form is best read as a historical transfer or inheritance, with its older language material carrying the main semantic load. As an adjective, it carries the sense "having or proceeding from extreme fanaticism; also, affected by rabies", with the older formation still helping explain the modern use. Relevant historical forms include Latin rabidus, Latin rabere, and Proto-Indo-European *rabh-. Derivative forms include rabidity, rabidness, rabidly; related words include rabies, rage, rave.