'fantasticalness' originates from English, specifically formed from the adjective 'fantastical' + the suffix '-ness', where 'fantastical' derives (via Middle French/Old French 'fantastique' and Late Latin 'phantasticus') from Greek 'phantastikos' (from 'phantazein', meaning 'to imagine' or 'to make visible'), and the suffix '-ness' comes from Old English '-nes(s)e' meaning 'state or condition'.

'fantasticalness' developed in Modern English by adding the noun-forming suffix '-ness' to 'fantastical' (which itself evolved from 'fantastic' ← Old French 'fantastique' ← Late Latin 'phantasticus' ← Greek 'phantastikos'), resulting in the modern noun meaning 'the state of being fantastical'.

Initially the root sense related to 'appearances, phantoms, or imagined visions' in Greek and early Romance uses; over time it shifted through senses of 'imaginary' and 'extravagantly fanciful' to the current meaning 'the quality of being fanciful or otherworldly'.