Why left foot kill foot right foot drill foot?

So I got this tattoo on my way out in 2012. About 8-9 of us did in my friend group. My son asked me today why it’s left foot kill foot and right foot drill foot. I for the life of me can’t remember what it stands for. Knocked my noggin a few two many times and I really don’t remember most of my time in service. It sucks how much I have lost. Pictures sometimes help but depressing….

(Yeah I know I got some nasty hobbit feet, haha was out in the garden pulling weeds getting eaten alive my the skeeters.)