It's like being a cock blocker, but for when you're trying to take a shit in a public restroom (office, restaurant, school, etc.). You go in there and there is someone in there and they can tell that you want to take a shit, but they stay in there to do their makeup, hair, take and excessively long time to pee or, most commonly, they are trying to wait you out cuz they have to shit too (inevitably resulting in a shit-showdown).
Person 1: man I was just trying to go poo and Jennifer came in and started to do her hair, so I just peed and left.
Person 2: yeah, she's a total shit blocker
Damn I got shitblocked 3 times in a row. I still haven't gone.
(noun) A person, thing, or event that prevents you from taking a much needed shit, and may or may not consequently cause you to shit in your pants. May also be used as a verb (shitblock). The person could be a cop who pulls you over for speeding - you were speeding because you had to shit so bad. It could also be an annoying guy or girl at a party who stops you to make a bad joke, or, for those in grade school or highschool, the teacher who will not let you use the restroom.
Dan was being quite the shitblocker today - I told him I had to use a restroom and that it was an emergency, but he just wouldn't let me go.
Man you got shitblocked today when that cop pulled you over for speeding.