One who refrains from drinking, smoking, and etc. in order to act as the brains of the situation.
Some may consider this person as "the lame one", when he/she (the designated thinker) really just want to be responsible and lookout for the well-being of others.
1. Drunk friend: "I am soooooo gonna go home with that sexy guy over at the bar."
Desginated Thinker: For goodness sake, he could be a serial killer and you're thinking about going home with him? Sit your ass down, you're not going anywhere.
2. Random drunk guy: Why isn't that guy over there drinking and having fun?
Random drunk woman: He's just being the designated thinker, making sure no one gets their ass into trouble.