1790, "infidel," earlier and also caffre (1670s), from Arabic kafir "unbeliever, infidel, impious wretch," with a literal sense of "one who does not admit (the blessings of God)," from kafara "to cover up, conceal, deny, blot out."
Technically, "a non-Muslim," but in Ottoman times it came to be used there almost exclusively as the disparaging word for "Christian." It also was used by Muslims in East Africa of the pagan black Africans; English missionaries then picked it up as an equivalent of "heathen" to refer to Bantus in South Africa (1731), from which use in English it came generally to mean "South African black" regardless of ethnicity, and to be a term of abuse at least since 1934.
mid-15c., "adherent of a religion opposed to Christianity," from Old French infidèle, from Latin infidelis "unfaithful, not to be trusted," in Late Latin "unbelieving" (in Medieval Latin also as a noun, "unbeliever"), from in- "not, opposite of" (see in- (1)) + fidelis "faithful" (from PIE root *bheidh- "to trust, confide, persuade").
Originally "a non-Christian" (especially a Saracen); later "one who does not believe in religion, disbeliever in religion generally" (1520s). Also used to translate Arabic qafir (see Kaffir), which is from a root meaning "to disbelieve, to deny," strictly referring to all non-Muslims but virtually synonymous with "Christian;" hence, from a Muslim or Jewish point of view, "a Christian" (1530s). As an adjective from mid-15c., "of a religion opposed to Christianity;" 1520s as "rejecting the Christian religion while accepting no other."