Source : Webster's Revised Unabridged Dictionary (1913)

Stillson wrench \Still"son wrench\
   A pipe wrench having an adjustable L-shaped jaw piece sliding
   in a sleeve that is pivoted to, and loosely embraces, the
   handle. Pressure on the handle increases the grip.

Source : WordNet®

Stillson wrench
     n : a large pipe wrench with L-shaped adjustable jaws that
         tighten as pressure on the handle is increased