What does Finny say the shirt stands for?

The shirt in an emblem to celebrate America's first bombing in Central Europe.

“I was reading in the paper that we bombed Central Europe for the first time the other day.” Only someone who knew Phineas as well as I did could realize that he was not changing the subject. I waited quietly for him to make whatever fantastic connection there might be between this and his shirt. “Well, we’ve got to do something to celebrate. We haven’t got a flag, we can’t float Old Glory proudly out the window. So I’m going to wear this, as an emblem.”