A jock or athletic type, or a man who is particularly macho or into sports, who has secret homosexual encounters. These are sometimes with other Frock Jocks, but more often the encounters are with a kid who is known to be gay, and is bribed to keep quiet. The term can also used to classify more specifically a jock who likes to "lift back his skirts" or "frock" a.k.a. receive anal sex.
Straight Kid (to friends): "Remember Patrick Geller? The football player back in high school in West Orange who all the girls were in love with? Did you hear he got photographed in drag up in that gay club in the heights?" Other Straight Friends:"Oh No Way!" "For real yo?" "That's mad crazy" Token Gay Friend: "Oh whatever, I always knew Geller was queer. The guy was such a frock jock."
i.e. "Greg" in Cruel Intentions