WordReference Random House Learner's Dictionary of American English © 2015
weak•en /ˈwikən/USA pronunciation v. 

    to (cause to) become weak or weaker: [no object]The animal weakened and eventually died.[+ object]A lack of resources will weaken our efforts.
weak•en•er, n. [countable]

Collins Concise English Dictionary © HarperCollins Publishers::

weaken /ˈwiːkən/ vb
  1. to become or cause to become weak or weaker

ˈweakener n

'weaken' also found in these entries:

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