Someone who impedes the flow of good times with his/her negative attitude, annoying behavior, and/or (but not limited to) incessant bitchiness towards anyone who is unfortunate enough to be in the company of such a killer of good moods, much like a tampon impedes the flow of...well you get the idea.
Todd: Anne was being such a fun tampon last night, we had to ditch her after the movie.
Matt: Guys this is the best birthday party I've ever had. Thanks so much for coming.
Anne: I've been having increased bowel movements recently; can someone show me where the restroom facilities are in case I get the runs again? Also, I haven't had sex in a while so I'm even more uptight than usual. Are you gonna eat any more of that?
Everyone and Their Mothers: What a fucking fun tampon.