'smoothen' originates from English, specifically the adjective 'smooth' plus the verb-forming suffix '-en' (a productive suffix used to form verbs meaning 'make X').

'smooth' comes from Old English 'smōþ' (or related forms) meaning 'smooth'; the verb form developed in Middle English by adding '-en' to produce 'smoothen', yielding the meaning 'to make smooth.'

Initially it meant 'to make physically smooth,' and over time it retained that meaning while also extending to figurative senses such as 'make easier' or 'make more even (e.g., data).'