monkey wrench

Short definition from Wiktionary

Noun

1. 1
   A wrench (spanner) with an adjustable jaw.
2. 2
   An unexpected or troublesome problem, obstacle or dilemma.
3. 3
   Alternative form of monkey wrench.

Verb

1. 1
   To apply a monkey wrench, e.g. to tighten or loosen nuts.
2. 2
   To introduce problems, to create unexpected trouble, to sabotage.