'egoistical' originates from Latin and New Latin roots: the Latin word 'ego' meaning 'I', combined with the suffix '-istic' (from Greek '-istikos' via Latin) forming an adjective meaning 'relating to the ego'.

'egoistical' developed through English formation from earlier borrowing of 'ego' (Latin) into English (as in 'egoism' and 'egoist'); the adjective forms 'egoistic' and the variant 'egoistical' arose in Modern English to describe characteristics of the ego or self-centered behaviour.

Initially it referred simply to things 'relating to the self' (from 'I'), but over time it came to mean specifically 'characterized by excessive concern for oneself' (i.e., selfish).