What is "petcock"

also pet-cock, 1864, from cock (n.2); the signification of the first element is uncertain.

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

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

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 a ball valve style petcock's is coarse, confining their use principally to on-off applications.