n. & adj.
n. 1 a person who does not believe in religion or in a particular religion; an unbeliever. 2 hist. an adherent of a religion other than Christianity, esp. a Muslim.
adj. 1 that is an infidel. 2 of unbelievers.
[ ME f. F infidèle or L infidelis (as IN-(1), fidelis faithful) ]