'antitheistical' is a compound formed in modern English from the prefix 'anti-' (from Greek 'anti-' meaning 'against') plus 'theistical' (from 'theistic', ultimately from Greek 'theos' meaning 'god').

'antitheistical' developed as an adjectival form related to 'antitheism' and 'antitheist' used from the 17th–18th centuries; the components come from Greek via Latin and modern English usage, producing the modern adjective 'antitheistical'.

Originally built simply to mean 'against theism' the word has retained that central sense; over time it has been used both descriptively (opposed to theism) and polemically (hostile or critical of religious belief).