To take large amounts of some sort of drug, legal or illegal, usually in pill or powder form.
"My back was hurting so I took four 30mm codeine tablets."
"Mate, that's double the stated maximum dose! You don't wanna be boshing codeine like that!"
Bosh its a method of describing a girl which provides oral sexual pleasure to differentpeople simply because she likes to, it can also be used as the oral sexual act itself