pet·cock /ˈpɛtˌkɑk/
pet, a. Petted; indulged; favorite; admired; cherished; as, a pet child; a pet lamb; a pet theory; a pet animal.
Some young lady's pet curate. --F. Harrison.
Pet cock. Mach. A little faucet in a water pipe or pump, to let air out, or at the end of a steam cylinder, to drain it. [Also spelled petcock.]
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petcock
n : regulator consisting of a small cock or faucet or valve for
letting out air or releasing compression or draining