You can picture someone holding a thing up and asking one question of it: did this really come from where it says it came from? A signature on a painting, a recipe from that village, a report of what happened — authentic is the answer 'yes, it traces back to the real source'.
From the word’s history
From Greek authentikos 'original, genuine, principal', from authentes 'one acting on one's own authority' (autos 'self' + hentes 'doer'). The old sense was 'authoritative, duly authorized'; the 'real, not fictitious' sense appears in English from the mid-14th century.
來自希臘文 authentikos「原來的、真的」，源頭是 authentes「憑自己的權柄做事的人」（autos「自己」＋ hentes「做的人」）。最早的意思偏向「有權威的、經過正式授權的」，中世紀英文（14 世紀中）之後才變成「真的、不是編出來的」。把「出自本人之手」這個畫面記住，四個用法就串起來了：畢卡索親手畫的（真跡）、那個地方原本的做法（道地）、事情本身留下的記錄（真實可靠）、跟原件做得一模一樣（逼真）。
Online Etymology Dictionary, https://www.etymonline.com/word/authentic
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原始詞典資料5 筆記錄
以下是外部詞典記錄的用法，摘錄並精簡格式，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
authenticadj/ɒˈθɛn.tɪk/
- 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
entertainmentlifestylemusicDesignating a mode having the final as the lowest note.
- 4
entertainmentlifestylemusicDesignating a cadence in which the dominant chord precedes the tonic.
- 5
obsoleteAuthoritative.
- [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.
摘錄自 英文維基詞典 English Wiktionary，已精簡格式；依 CC BY-SA 4.0 授權使用，改動後的內容同樣以此授權提供。