I Just Pissed My Pants
It implies something is extremely funny, like ROTFL, there are several variations of this abbreviation including PMP (Pissed My Pants) and JPMP (Just Pissed My Pants) and PIMPL (Peeing In My Pants Laughing).
Online jargon, also known as text message shorthand, used in texting, online chat, instant messaging, email, blogs, and newsgroup postings, these types of abbreviations are also referred to as chat acronyms.
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NetLingo Classification: Acronyms and Text Message