verb (used with object)

  1. to make flat.
  2. to knock down: The boxer flattened his opponent in the second round.

verb (used without object)

  1. to become flat.

Verb Phrases

  1. flatten in, Nautical. flat1(def 61).
  2. flatten out, Aeronautics. to fly into a horizontal position, as after a dive.


  1. (sometimes foll by out) to make or become flat or flatter
  2. (tr) informal
    1. to knock down or injure; prostrate
    2. to crush or subduefailure will flatten his self-esteem
  3. (tr) music to lower the pitch of (a note) by one chromatic semitoneUsual US word: flat
  4. (intr foll by out) to manoeuvre an aircraft into horizontal flight, esp after a dive

late 14c., “to prostrate oneself,” also “to fall flat,” from flat (adj.) + -en (1). Meaning “to make flat” is 1620s. Related: Flattened; flattening.

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