The phrase "bomb tesuji" is a joke pertaining to the game of Go. For those who don't play the game: "tesuji" is a Japanese term which means something to the effect of "clever move." They're usually the moves that make or break a game, and often the one move that can save a situation. Different kinds of tesujis have different names, such as "eye-stealing tesuji," and "squeeze tesuji," so "bomb tesuji" sounds like it might be a real one, which is why it's funny to suggest it to an opponent who doesn't know the joke.

Anyway, the joke. Some players have the bad habit of holding a Go stone in their fingers and dangling it above the goban (board) while thinking about their next move. This is a bad habit for a number of reasons, but one of them is that the stone occasionally slips from the person's fingers and lands on the board, sometimes disturbing the other stones. So the joke is that if your situation is totally hopeless, you might consider playing the "bomb tesuji," that is, doing this deliberately and messing up the stones in a key area of the board so that the game can't go on, and pretending that it was an accident.

Disclaimer: It's a joke. It would be extremely poor form to actually do this in a game of Go, although it is, as I said, a funny and mild form of trash talk to suggest it to your opponent as his best option.

Addendum: I did this for the first time last night (by accident, of course). It wasn't from dangling the stone over the goban, though, since that's one of the few bad habits that I don't possess. Rather, it happened because I got so excited over a good move that I was about to play that when I whipped the stone out of its bowl, it flew from my fingers and smashed into a formation on the far side of the goban. Fortunately, we were able to agree on how to reconstruct the situation. Since this is slightly different from the standard bomb tesuji, perhaps it needs a new name. Cruise missile tesuji sounds about right.