http://www.lostcherry.com/viewimage.php?u=163773&albumid=0&i=1420980233
The top character 太 means “very, too, much; big; extreme”, and the bottom one 屁 means “break wind; fart; buttocks”.
In Chinese slang, especially in internet lingo, 太屁 means “lame” or "pathetic".
Update: August 27, 2006 - According to the original website, the tattoo is supposed to be "fat ass".
If that is true, the top character should be 大, not 太.