Another one in the casket basically means going to kill you or to GET killed. My mum told me that when I was a little girl. So, the definition of it is I'm going to kill you or going to kill you.
Random guy: Dude, my grandma's about to die in a year.
Another random guy: Oof, another one in the casket.
Jayson: I have a bonner for you parker
Parker: alright lets have.... YOU GET WHAT IM SAYING
when a guy has a bonner for another dude