Latin: "Of an illegal cause there can be no lawsuit."
In other words, if one is engaged in illegal activity, one cannot sue another for damages that arose out of that illegal activity.
A example is an injury suffered by a passenger in a stolen car, which that passenger knew to be stolen and was a free participant in the joyriding.
If vehicle crashes injuring the passenger, there is no action in tort against the driver under the ex turpi causa non oritur actio principle.