• authentical, authentick, authenticke, authentique (all archaic)

From Middle English authentik, from Old French autentique, from Latin authenticus, from Ancient Greek αὐθεντικός (authentikós, “principal, genuine”), from Ancient Greek αὐθέντης (authéntēs, “lord, master”). Doublet of effendi.

• (UK) IPA^(key): /ɒˈθɛn.tɪk/
• (US) IPA^(key): /ɔˈθɛn.tɪk/
• (cot–caught merger) IPA^(key): /ɑˈθɛn.tɪk/

• Audio (Southern England):

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authentic (comparative more authentic, superlative most authentic)

1. Of the same origin as claimed; genuine.

   The experts confirmed it was an authentic signature.

2. Conforming to reality and therefore worthy of trust, reliance, or belief.

   an authentic writer; an authentic portrait; authentic information

   The report was completely authentic.

3. (music)
   1. Designating a mode having the final as the lowest note.
   2. Designating a cadence in which the dominant chord precedes the tonic.
4. (obsolete) Authoritative.
   • 1641 June or July, [John Milton], Of Prelatical Episcopacy, and Whither It may be Deduc’d from the Apostolical Times by Vertue of Those Testimonies which are Alledg’d to that Purpose in Some Late Treatises: […], London: […] R[ichard] O[ulton] & G[regory] D[exter] for Thomas Underhill, […], →OCLC, page 11:

     [T]here they find Ignatius, and then they beleeve him not for his ovvn Authority, but for a truths ſake, vvhich they derive from elſ vvhere: to vvhat end then ſhould they cite him as authentick for Epiſcopacie, vvhen they cannot knovv vvhat is authentick in him, but by the judgement vvhich they brought vvith them, and not by any judgement vvhich they might ſafely learne from him.

• (of the claimed origin): genuine, real, bonafide, bona fide, unfaked
• (conforming to fact): reliable, trustworthy, credible, unfaked

• (antonym(s) of “not of the claimed origin”): non-authentic, nonauthentic, phony, fake; ingenuine, inauthentic, unauthentic

• “authentic”, in OneLook Dictionary Search.
• authentic in Keywords for Today: A 21st Century Vocabulary, edited by The Keywords Project, Colin MacCabe, Holly Yanacek, 2018.
• William Dwight Whitney, Benjamin E[li] Smith, editors (1911), “authentic”, in The Century Dictionary […], New York, N.Y.: The Century Co., →OCLC.