A girl who probably sits next to you in some class and your first impression of her is that shes probably too nice or well behaved to do anything freaky like let loose a rumbling rumplestiltskin or shout "streaking windwardly scuzzbucket!!!!"
But then you go to a party and she happens to be the only sober one there and shes still crazier than Stephanie dry humping the ladder that you brought into the living room.
Alex - Hey Sharlene ... How's life?
Sharlene - ... STREAKING WINDWARDLY SCUZZBUCKETS LOLOLOLOOLOL!!!! *Straddles ladder violently*
Alex - 0.o ( . Y . )
Stephanie - I CAN DO IT BETTER!!! LOOK MOM! NO HANDS!
Sharlene - Pthltpbth
Often used in such cartoons as the famous "Calvin and Hobbes" to illustrate a farting noise made with the mouth. It is also the best way to get whoever is talking to you to stfu.
Steph - Liek omg i today i wuz awl liek deezinarr perfyoome shawpeeng wen dis hoez cumz all up in mah grill liek OMG can i haff ur autograf plz?!?! s o i r just liek mmkah cuz i r prettier than u r lolololol
You - PTHLTPBTH!!!!!!!
Steph - ... LIEK OMG SO IMMATU
Steph - ... *Storms off muttering about how much better than everyone else she is.*