The process of celebrating the birth of a baby. Traditionally taking its name from the Christian baptismal rite, during which the head of a baby would be wet with blesed water, the phrase now commonly relates to the consumption of large amounts of alcohol as a celebration of the new arrival.
The actual process of wetting the baby's head is far more fun before the baby is born.
Bloke 1: check out the cans on that barmaid.
Bloke 2: Dude, you're married.
Bloke 1: Yeah, but...
Bloke 2: You're wife just had a kid.
Bloke 1: Yeah, well..
Bloke 2: We're here celebrating, to wet the baby's head.
Bloke 1: True. But she has got really nice cans.