There were two stories with the idiom 'wet the baby's head' in the headline in the Daily Mail today. In one the baby belonged to Liverpool footballer Glen Johnson, and the other referred to the actor Orlando Bloom's new arrival.
Not that the babies were involved in these activities. To wet the baby's head means 'to drink to celebrate the birth of a child' (OED definition) and was first recorded in 1881.
The OED has other expressions using 'wet' in the sense 'have a drink (or several!)' There's 'to wet a commission' (in the army or navy), 'to wet the baby's eye', 'to wet the other (or t'other) eye' (to drink one glass after another), and 'to wet one's whistle/ weasand (= throat)/ mouth/ beard/ beak or clay'.
Jonathon Green's Dictionary of Slang has 'wet-thee-through', meaning 'gin'. I forgot to include this nickname when I wrote on the different slang words for gin a couple of years ago.