'assuetude' originates from Latin, specifically the word 'assuetudo', where the verb 'assuescere' meant 'to accustom'.

'assuetude' passed into Middle English and was also present in Old French as 'assuetude' before becoming the modern English word 'assuetude'.

Initially it meant 'the process of becoming accustomed' or 'accustoming', and over time it has retained this central sense, coming to denote both the process and the resulting state (habit or familiarity).