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.