1. Originally applied to males. Any act blocking the pursuits of another through alphamale/dominant/dickish behavior.
Originally observed and defined in Moore Hall, 4th Floor, College Station Texas while watching a porn. A male actor was lining up his approach on a female actress in doggy style scene when Peter North pushed the lesser known actor out of the way and took "pole" position for himself. Henceforth this was known as the "cockblock". The term has grown from there.
I got totally cockblocked at that orgy on Saturday and ended up with a dwarf exchange student.