designated cockblocker

the person who stays sober at a party, club, etc. and keeps his/her friends from hookin up with grenades, landmines, or just someone ( or people) they shouldn`t

Ronnie: Yo bro mad people are hittin up karma tonight, lets go.

Mike: Cool

Ronnie: But bro, Sam`s gonna be there

Mike: That`s cool, i`ll be your designated cockblocker