The female version of jacking off derived from Jack and Jill. Jack for the boys, Jill for the girls.
I walked in on Marina jilling off while she was watching porn. I'm not sure whether she was extremely startled at the disturbance, or if she jilled her panties! It was so loud.
The act of hanging with, walking in hallways or chatting on facebook with the members of JIL. Often includes conversation about interesting and philosophical topics.
Guy 1: Hey, What are you doing?
Guy 2: Just jilling!
Guy 1: *confused look*