FAMILIAR (through the Fr. familier, from Lat. familiaris, of or belonging to the familia, family), an adjective, properly meaning belonging to the family or household, but in this sense the word is rare. The more usual meanings are: friendly, intimate, well known; and from its application to the easy relations of intimate friends the term may be used in an invidious sense of "free and easy" conduct on the part of any one not justified by any close relationship, friendship or intimacy. "Familiar" is, however, also used as a substantive, especially of the spirit or demon which attended on a wizard or magician, and was summoned to execute his master's wishes. The idea underlies the notion of the Christian guardian angel and of the Roman genius natalis (see Demonology; Witchcraft). In the Roman Church the term is applied to persons attached to the household of the pope or of bishops. These must actually do some domestic service. They are supported by their patron, and enjoy privileges which in the case of the papal familiars are considerable. "Familiars of the Holy Office" were lay officers of the Inquisition, whose functions were chiefly those of police, in making arrests, etc., of persons charged.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)