'antiegoistically' originates from combining the prefix 'anti-' (from Greek 'anti' meaning 'against, opposite') with the Latin root 'ego' ('ego' meaning 'I'), the adjective-forming suffix '-istic' and the adverbial suffix '-ally'.

'antiegoistically' was formed in Modern English by attaching 'anti-' to 'egoistic' (itself from Latin 'ego' > English 'ego' + '-istic'), producing 'antiegoistic' (adj.), and then adding '-ally' to create the adverb 'antiegoistically'.

Initially formed to express opposition to 'egoism' (literally 'against ego'), it has come to mean specifically 'in a way that is not egoistic; selflessly' in modern usage.