What does a knife going through a skull tattoo symbolize?

The knife going through the skull was a very popular tattoo during the 50's and early 60's. The most popular area to place the tattoo in on the inner forearm. There isn't any symbolism behind it other than it looking tough. It was a popular tattoo design of the past and mostly attained by bikers so it could be seen while the rider's arm was stretched out, griping the motorcycle handlebars.