The Collaborative International Dictionary of English v.0.48:

monkeywrench \monkeywrench\, monkey wrench \monkey wrench\n.
   A wrench or spanner which has one fixed and one adjustable
   jaw.

   Syn: monkey wrench.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

monkey-wrench
    n 1: adjustable wrench that has one fixed and one adjustable jaw
         [syn: monkey-wrench, monkey wrench]