'amerism' originates from 'American English', specifically the word 'American', where 'Ameri-' referred to 'America' and '-ism' denoted 'a distinctive practice or system'.

'American' transformed into the term 'amerism' to specifically denote expressions unique to American English.

Initially, it meant 'a characteristic of America', but over time it evolved into its current meaning of 'a term or expression unique to American English'.