Sentence:  [Niccolò] Machiavelli had received an early lesson in the value of <WSD>addling</WSD> men's brains. ... [A] talent for <WSD>addling</WSD> men's brains is part of the armoury of any successful prince .... 
Meaning of the word marked with <WSD> tags:  To make addle; to grow addle; to muddle