A sexual act/act of disrespect where one ejaculates into an instrument (typically brass or woodwind), then blows said ejaculate into the face of an unsuspecting listener.
"The band director is really pissing me off today."
"Let's give that motherfucker a creampied piper. That'll teach him."
Short-hand for the word floccinaucinihilipilification, meaning the estimation of something as valueless (encountered mainly as an example of one of the longest words in the English language).