Other forms: flattened, flattening, flattens
To flatten something is to make it level, or to squash it down. A rolling pin, a steamroller, a hand across a map.
It works both ways round without changing form, since you flatten the dough and the dough flattens. Informally it means to knock somebody down, as in he was flattened by the tackle, and a storm can flatten a crop or a building. Charts made a new use of it, since flattening the curve means slowing a rise. It belongs to the -en family beside widen, deepen and sharpen.
Proto-Indo-European
*pleth₂-
Middle English
flat
English
flat
English, today
flatten
From flat + -en. flat comes from Middle English flat. From Middle English flat, a borrowing from Old Norse flatr (compare Norwegian and Swedish flat, Danish flad), from Proto-Germanic flataz, from Proto-Indo-European pleth₂- (“flat”); akin to stq flot (“smooth”), German Flöz (“a geological layer”), Ancient Greek πλατύς, Latvian plats, Sanskrit प्रथस् (“extension”). a doublet of plat and pleyt. The noun is from Middle English flat (“level piece of ground, flat edge of a weapon”), from the adjective.
Etymology from Wiktionary, CC BY-SA