'egoistically' derives from the adjective 'egoistic', which in turn comes from the noun 'ego' (Latin 'ego' meaning 'I') combined with the suffix '-istic' (forming adjectives meaning 'relating to').

'ego' (Latin) gave rise to French 'égoïste' and English 'egoist' in the 18th–19th centuries; from 'egoist' came the adjective 'egoistic', and the adverbial form 'egoistically' was formed by adding the adverbial suffix '-ally'.

Originally related to the Latin sense of 'I' or the self ('I'-centered), the term evolved to mean specifically 'concerned chiefly with one's own interests' and now commonly denotes a selfish manner ('in a selfish way').