This achievement is called "Face Hoff"
To earn this achievement you have to defeat Princess Kenny while wearing the David Hasselhoff head which can be purchased at Tom's Rhinoplasty for $175. It is the last item on the list, labelled as "the hasselhoff"
I'm rewriting this note for clarification.
After popping this achievement i started a second playthrough to go for the AIDS/ 2Girls achievements. Throughout the whole second playthrough no matter what wig/helmet/make up etc i equipped it wouldn't show on my character. It was treating my game like i was still wearing the hasselhoff from my first playthrough. At the end Aids popped but 2 girls didnt. so i started a third playthrough and still was having this problem. I deleted the game file from my hard drive and started another playthrough and no more problems.
I haven't tried all 3 at once since since i already earned them but i've heard many people having no problem. I just recommend that you save this for last to avoid this problem, even if its all 3.