May refer to two things in English slang (this is true for American and subsequently (and sadly) Canadian...unsure for British). /msg me if yo know ;)

1. (vulgar) A synecdoche where the head of the penis represents the whole penis, thus the expression "Giving head." This in turn creates a metonymy which attributes "head" to blowjobs.

"Dude, he gives sloppy head! I can do way better."

2. The foam from the top of beer poured into a glass. This is usually used when there is a considerable amount of foam accumulated.

"Dude, this beer is ALL head! Wtf?"

"But...but...I don't understand...this is factual.../me runs away and cries."