late 14c., "to prostrate oneself," also "to fall flat," from flat (adj.) + -en (1). Transitive meaning "to make flat" is 1620s. Related: Flattened; flattening.
flatten a road
flatten your stomach with these exercises
The landscape flattened
flatland
flatline
flatly
flatness
flats
flatten
flatter
flatterer
flattering
flattery
flat-top
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