The Collaborative International Dictionary of English v.0.48:

Monkey \Mon"key\, v. t. & i.
   To act or treat as a monkey does; to ape; to act in a
   grotesque or meddlesome manner.
   [1913 Webster]

   To monkey with, To monkey around with, to handle in a
      meddlesome manner. [Colloq.]
      [1913 Webster +PJC]