infantile

the word 'infantile' is a direct borrowing from late latin infantilis, which itself is derived from the classical latin noun infans. this noun is a compound formed from the negative prefix in- ('not') and the present participle fans ('speaking'), from the verb fari ('to speak'). thus, its literal origin means 'one who does not speak,' referring to a very young child. the suffix -ilis was a productive latin suffix for forming adjectives. over time, the meaning extended from 'pertaining to an infant' to also describe behavior or qualities characteristic of an infant, leading to its modern, sometimes pejorative, sense of 'childishly immature.'

Structure: infant + ile infant (from latin infant-, infans meaning 'unable to speak, young child'), ile (from latin -ilis, an adjectival suffix meaning 'pertaining to, capable of')