'antialien' is a modern English compound formed from the prefix 'anti-' (from Greek 'anti-' meaning 'against') and 'alien' (from Latin 'alienus' meaning 'other, belonging to another').

'alien' comes from Latin 'alienus' which passed into Old French and then Middle English as 'alien'; the prefix 'anti-' comes from Greek ἀντί via Latin/modern adoption as a combining form; the compound 'anti- + alien' was formed in modern English to mean 'against alien(s)'.

Initially the elements meant 'against' (anti-) and 'other/foreigner' (alienus); over time the compound has been used to denote opposition either to foreign people (immigrants) or to extraterrestrial beings, with nuances depending on context.