Short for chill session. Describes any amount of time spent with 1 or more people, although usually not large groups. A chillsesh is typically either last-minute or unofficially planned, and sometimes completely spontaneous. Because of this, a chillsesh can last anywhere from half an hour to several hours.
"Me and Jim were just walking around town and ran into Sarah, so we ended up having a chillsesh for a little while."
"Hey, you doin anything after school?"
"Chillsesh at my house?"
"So did you end up hanging out with her?"
"We didn't really hang out. We just had like a one-hour chillsesh, but then she had to go."