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.
Dictionary.com Unabridged
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
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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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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Computing Dictionary

flatten definition

To remove structural information, especially to filter something with an implicit tree structure into a simple sequence of leaves; also tends to imply mapping to flat ASCII. "This code flattens an expression with parentheses into an equivalent canonical form."
[Jargon File]

The Free On-line Dictionary of Computing, © Denis Howe 2010 http://foldoc.org
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Example sentences
There are some things so elastic that even the heavy roller of democracy cannot
  flatten them altogether down.
Trim off fat and skin from three pounds of beef cut from centre of fillet and
  flatten with a broad-bladed cleaver.
The flowers, funnel-shaped when they open in the morning, flatten as the day
  progresses and are dropped the following day.
Hurricanes bring winds and slashing rains that flood streets, flatten homes,
  and leave survivors struggling to pick up the pieces.
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