"shaking my penis", similar to "shaking my head" but simply with a different body part. Used to show disbelief, dissatisfaction or extreme satisfaction about a topic.
Shit My Pants. In rebellion to the absolute overindulgence of LOL in texts, twitter, email, facebook etc, SMP is to be used in its place... If people were laughing out loud as often as they use the term there would be no war, the world may in fact end because no one would be doing anything but laughing out loud. Therefore SMP is to replace LOL. And only to be used if you are in fact shitting your pants.
Over Text Message-
Kristie: OMG I'm at the beach and i farted so loud the ground vibrated and fellow beach goers left bc they thought a storm was coming
Shit My Pants laughing. Due to the overuse and abuse of laughter related, played out texting abbreviations (e.g. LOL, ROTFL, Ha, etc.), the holy grail of laughing, but ACTAULLY laughing is, Shit My Pants laughing. This is elegantly denoted with the anecdote, SMP.
Mike: Oh man, WTF Parker. You banged my mom last night?
Parker: SMP yeah, I was drunk and needed a booty call.