LATIN DICTIONARYLexicon Latīnum
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“familiar” in Latin

The English word familiar itself goes back to Latin familia, through familiāris.

Riddle & Arnold A Copious and Critical English–Latin Lexicon, 1864

familiar
Pertaining to a family; domestic, familiaris: domesticus. (The words are found in this connection and order.) domesticus ac familiaris.
Intimate, familiaris: domesticus: intimus (SYN. in INTIMATE). Anybody’s familiar friend, alicujus or alicui familiaris: familiaris amicus (Plin.): a mast familiar friend, familiarissimus (alicujus): qui est ex alicujus domesticis atque intimis familiaribus (Cic.). [Vid: INTIMATE.]
Intimately known, well-acquainted, familiaris (e.g., hunc, qui familiarior nobis... est. Demosthenem, Cic.; vox auribus meis familiaris, Petronius): bene notus (well-known). ☞ To translate “with which we are familiar,” say qui nobis est familiaris, bene notus, etc.: to be familiar with anything, aliquid mihi familiare or bene notum est: familiariter novi aliquid (Quint.). [Vid: ACQUAINTED.]
Affable, intimate (in manner) with those beneath us, communis (e.g., communis infimis, par principibus, Nep.). [Vid: AFFABLE.] To be familiar with anybody, familiariter loqui cum aliquo (Cic.): familiarius vivere cum aliquo (with reference to the terms on which one lives). [Vid: AFFABLE.]
Simple, not elevated, etc., with reference to style, ad sensum popularem vulgaremque or ad commune judicium popularemque intelligentiam accommodatus (popular; ☞ not popularis or familiaris in this sense, though familiaris interpretatio has been commonly used, and lately even by Orelli): familiariter scriptus (in the style of a friend writing naturally and confidentially to a friend; Cic.).
A familiar spirit, *umbra, quae alicui adest: genius alicujus, or (after Servius’s definition) naturalis deus alicujus.

Quick answer

What is the Latin word for familiar?
The usual Latin words for “familiar” are familiāris, domesticus, interior; also commūnis. The right choice depends on the sense; see the distinctions below.

Entry from J. E. Riddle & T. K. Arnold, A Copious and Critical English–Latin Lexicon (rev. C. Anthon, New York: Harper, 1864), digitised by Fergus J. P. Walsh, CC BY-SA 4.0; Latin markup adapted by latindictionary.co.