'authenticness' originates from the adjective 'authentic' (itself ultimately from Greek) combined with the English noun-forming suffix '-ness'. 'Authentic' comes via Late Latin 'authenticus' from Greek 'authentikos', where 'authent-' (from 'authentēs') meant 'real' or 'original', and the suffix '-ness' (Old English '-nes(s)') denotes 'state or quality'.

'authenticness' is an English formation created by attaching the native suffix '-ness' to 'authentic'. The adjective 'authentic' entered English via Old French and Latin from Greek in the Middle to Early Modern English period; the '-ness' formation is a traditional Germanic/Old English way of creating abstract nouns, so the combined form arose in Modern English as an alternative to the Latinate noun 'authenticity'.

Initially, the elements conveyed 'the state or quality of being authentic' and this core meaning has remained; over time the Latinate noun 'authenticity' became the standard form in formal usage, while 'authenticness' remained a less common or nonstandard variant.