This is the act of hitting your testicles in somebodys face. Sacklash can also mean when you hit your dick in someones face and it leaves a mark such as a cum mark caused by pre-come from the penis.

The art of performing the perfect sack-lash is to swing your penis or balls quickly in a sideways motion until you hit the other persons face, or any other part of the body. If done while pre-cumming or upon ejaculation, then the perfect sack-lash mark can be achieved

When you eat dirt for the first time and leave your family to build a shack in the woods