The Collaborative International Dictionary
Pet cock
pet \pet\, a. Petted; indulged; 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. [Perh. for petty 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.
Usage examples of "pet cock".
Knowing that I didn't have his price and was unlikely to pay it if I did, he planned on keeping his pet cock until it died of old age.