petcock

Etymology
Apparently from, because of its size.

Noun

 * 1) A small valve, spout, or faucet operated by hand, usually used to release pressure or drain fluid.
 * 2) * 2010, International Fire Service Training Association, Essesntials of Fire Fighting and Fire Department Operations, 5th Edition, p. 780:
 * The shutoff valve (petcock) is usually located on the gas pipe on the supply side of the meter.
 * The shutoff valve (petcock) is usually located on the gas pipe on the supply side of the meter.

Translations

 * Bulgarian: кран за източване
 * German: Benzinhahn, Ablaufhahn,, Entlüftungshahn, Entlüftungsventil, Kühlerablasshahn, Spülstutzen, Entlastungsventil