A cross between two similar phonetic spellings of expressions: Whee, a high pitched glee; and Bwahaha, an evil and maniacal laugh. This combination results as a mischievious shout of joy. May be lengthened and/or have "hehe" added to the end of it for emphasis.
"Bweeee!"
"What are youup to this time?"
"Nooooothing."