Draining valve

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

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.)

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

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 ...

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.