My friend thinks he's a great pool player.

This joke is a classic example of wordplay, specifically a pun, that hinges on the double meaning of the word "sinking." In the game of pool, "sinking" a ball refers to successfully pocketing it, which is the goal of every player. However, "sinking money into something" is an idiom that means investing a significant amount of money, often into something that isn't yielding a good return or is simply a costly hobby. The humor arises from the unexpected twist where the friend's supposed skill in "sinking" things actually refers to his financial losses.

The world of pool and billiards has a long history, dating back centuries, and it's often associated with friendly competition, but also with wagering. Whether it's paying for table time at an arcade or bar, or placing bets on individual games, playing pool can certainly become an expensive endeavor. So, if someone consistently plays poorly, they are quite literally "sinking" their hard-earned cash into the game, making the punchline a relatable and amusing observation for anyone who's ever found themselves on the wrong end of a pool cue – or a losing streak.