verb (used with object)
to make flat.
to knock down: The boxer flattened his opponent in the second round.
verb (used without object)
to become flat.
Verb phrases
flatten in, Nautical, flat1 ( def 61 ).
flatten out, Aeronautics. to fly into a horizontal position, as after a dive.

1620–30; flat1 + -en1

flattener, noun
overflatten, verb (used with object)
unflattened, adjective

2. ground, fell, prostrate, deck, floor.
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
flatten (ˈflætən)
vb (foll by out)
1.  (sometimes foll by out) to make or become flat or flatter
2.  informal (tr)
 a.  to knock down or injure; prostrate
 b.  to crush or subdue: failure will flatten his self-esteem
3.  (tr) music Usual US word: flat to lower the pitch of (a note) by one chromatic semitone
4.  to manoeuvre an aircraft into horizontal flight, esp after a dive

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

late 14c., "to prostrate oneself," also "to fall flat," from flat + -en (1). Meaning "to make flat" is early 17c. Related: Flattened; flattening.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences for flattened
The forehead is flattened, and the bill is strong and pointed.
The monociliated cells of the dorsal epitheloid are flattened and contain lipid
It has a thin green skin fused with the white flesh, and a single large
  flattened pip.
In many varieties the fruit is flattened on one or two sides.
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