Way back in the 60's it was a common phrase used for an erection. It was used in reference to the teacher on the Archie cartoon series, Miss Grundy. If your penis was Really hard, it was called a "Grundy" because it pulled so hard on your skin that the leaders(tendons) in your neck showed like the teacher on the Archie cartoons.
That hot chick over there in the string bikini is giving me a Grundy!!