Fin is a Czech slang word used to describe a gravity bong made of any water bottle and a small glass pipe that you can buy any place you can buy cigs as well, that you can find mostly in the cities of Moravia, where broke teenagers don't have the
money to spend on
weed so they smoke these because they use a small amount of
weed and get you fucked up. The taste is very bitter but, it is economical and fast, I myself can smoke a Fin, having the bottle with water and a hole and a cap with a pipe and
weed ready, in about 15 seconds.