I belong to PLURK which is a microblog similar to Twitter. Some of my "friends" (contacts) post in Chinese. Occasionally out of curiosity I paste it into translate.google.com
This screen name 屁屁杰分享 translates oddly as Ass Kit Share but when I break it down to individual characters then of course 屁 by itself translates as "breaking wind" to avoid the cruder word. Years ago a waiter in a Chinese restaurant taught me that word and now I realize that he was accurate. Apparently, according to the Google translator, the combined 屁屁 means buttocks. I must do some more research.

As a kindergarten teacher in China, I can attest that 屁屁 is a diminutive for rump. My students love it twice as much because of what it means in English as well. 打屁屁，打屁股，and 打屁 all indicate the passing of gas, though it can also be used for spanking (often with a possessive in the middle, i.e. 打我的屁股).