Heh, usually Yui is the only one who wears these "word shirts", and here she's the only one whose shirt I can't read.

Looks like 大黒柱（だいこくばしら）. Rikaichan says it can mean things like backbone(of a family) and main man.

That sounds right, but I'm still not confident enough to make a note.

That checks with my sources too, so I think a note is warranted. The slangy "main man" meaning is probably what Yui's going for.