Slang for defining a decoy, that appears underage to entrap older persons, most notably gay men. Used more when Homosexuality was illegal.
During the time when homosexuality was illegal, many vice departments used young looking officers to entrap gay men. These decoys were referred to as 'fairy hawks'.
It was also used to describe 'gay bashers' who would try to lure gay men, for the purpose of beating them up, or for use in blackmail/extortion schemes.
Noteworthy: Mainly used for entrapment purposes, as homosexual acts, with any person was illegal at the time. Fairy Hawks could be of different types, not just young looking in characteristics.