Regulator consisting of a small cock or faucet or valve for letting out air or releasing compression or draining

• Draining valve
• Small faucet

Wiktionary Word definitions in Wiktionary
n. A small valve, spout, or faucet operated by hand, usually used to release pressure or drain fluid. (from 19th c.)

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
also pet-cock , 1864, from cock (n.2); the signification of the first element is uncertain.

Wikipedia Word definitions in Wikipedia
A petcock is a small shut-off valve used to control the flow of liquid or gas . Historically, petcocks were threaded valves controlled by a butterfly handle; modern petcocks are typically ball valves . Compared to the progressive control of a gate valve ...

WordNet Word definitions in WordNet
n. regulator consisting of a small cock or faucet or valve for letting out air or releasing compression or draining