Generally, an instance where luck works against you in a game at the worst times or with no apparent justification. This can include tabletop rpgs or video games. A combination of the words bullshit and critical hit, usually shorted to crit.
Originally from Team Fortress 2, which uses a random critical hit system, and referring to the Soldier's devastating crit rockets in particular.
One second I'm an overhealed Demoman, and the next a Soldier with 5 hp one-shots me as he rounds the corner. It's pure bullcrit.