A large pipe wrench with L-shaped adjustable jaws that tighten as pressure on the handle is increased WordNet 3.0 & Open English WordNet

An adjustable wrench (British: spanner) with a toothed jaw for gripping pipe, frequently used by a plumber or pipe fitter, to loosen and tighten pipes with threaded connections, designed such that any forward pressure on the handle tends to pull the jaws (otherwise slightly mobile in the tool plan) tighter together. OmegaWiki