An acronym for "Wrong Person". When someone says something to another person, but at the time, did not notice that they were talking to someone else, who they didn't mean to speak to. Often used in instant messengers.
"An example situation in an instant messaging room:"
SomeGuy: Hey, remember when I laughed at that guy with the giant tumor on his head?
SomeChick: Umm, what?!
SomeGuy: Sorry, WP.