'archaicness' is formed in English by adding the suffix '-ness' to 'archaic' (adjective), which yields a noun meaning 'state or quality of being archaic.'

'archaic' comes from Late Latin 'archaicus' and via French from Greek 'archaîkós' (ἀρχαϊκός), from 'archaîos' (ἀρχαῖος) meaning 'ancient'; the English noun was later extended with the native English suffix '-ness' (from Old English '-nes(s)e') to form 'archaicness.'

Originally related to 'ancient' or 'belonging to an earlier period,' the sense has broadened in modern English to emphasize 'old-fashioned' or 'out of current use' (often with a stylistic or evaluative nuance).