'imperturbable' originates from Latin, specifically the word 'imperturbabilis,' where 'im-' (variant of in-) meant 'not' and 'perturbare' meant 'to disturb.'

'imperturbabilis' passed through Late Latin/Medieval Latin and entered English (via French influences) as 'imperturbable,' eventually becoming the modern English adjective 'imperturbable.'

Initially it meant 'not able to be disturbed' in a literal and figurative sense; over time this meaning has remained stable and now commonly describes a person's calm temperament ('not easily upset or excited').