"Snake heads" is a sex act involving a person kneeling in between two people and blowing then both (one penis in each hand) then making the "Snake heads" kiss by touching the two penises together.
A slight shake of the head, meaning you screwed up so bad, that the other side can only shake their head. Or, other side is fed up with your shit.
Example 1:
Bob: DUDE! I ate a porcupine -- when it was still ALIVE!
Freddy: *Shake of the head*
Example 2:
Bob: Dude, I climbed Mt. Everest, pulled of my helmet, para-glided down, and landed on my head! And the only evidence I have is this small scratch on my elbow!
Freddy: *Shake of the head*