Generally used by rabid anime fans who can't spell bishounen so they shorten it.
Originally from the Japanese word: Bishounen Bi, sho, nen.
Each character meaning: Beautiful small year; respectively, roughly meaning "pretty boy"
Pretty girl is somewhat different: Bi sho jou.
Again each character meaning: Beautiful small woman, respectively. Roughly meaning "pretty girl"
He is so bishie.
Contrary to popular belief, this term isn't really a shortened form of "bishounen". It is used to refer to boys under the age of fourteen.