'assumpsit' originates from Medieval Latin, specifically the word 'assumpsit', where the prefix 'ad-' (assimilated to 'as-') meant 'toward' and the root 'sumere' (from Latin 'sumere') meant 'to take' or 'to take up' (from 'assumere' = 'to take up, undertake').

'assumpsit' changed from Medieval Latin legal/formal use 'assumpsit' into Anglo-Norman and Middle English legal usage (recorded in writs and pleadings as 'assumpsit') and eventually became the modern English legal noun 'assumpsit'.

Initially it meant 'he has undertaken' as a past-tense verbal form, but over time it evolved into its current meaning of 'a common-law action for breach of contract or a broken promise'.