'flatten' originates from English, specifically from the adjective 'flat' plus the verbalizing suffix '-en', where 'flat' meant 'having a level surface'.

'flatten' developed by adding '-en' to 'flat' in Middle/Early Modern English; the noun 'flattening' arose by adding the suffix '-ing' to form the action or result, and 'flattenings' is the plural of that noun.

Initially it meant 'to make or become flat'; over time the core meaning has remained but extended to figurative and technical senses (e.g., 'flatten the curve').