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A small valve or faucet used to drain or reduce pressure, as from a boiler.
(Mechanical Engineering) a small valve for checking the water level in a steam boiler or draining condensed steam from the cylinder of a steam engine
[C19: from pet1 or perhaps French pet, from péter to break wind + cock1]
a small valve or faucet, as for draining off excess or waste material from the cylinder of an internal-combustion engine.