When you have a session of anything! For Example:
Hanging out with friends,
drinking with friends,
fucking your girlfriend.
Guy 1 : I'm off for a seshwin!
Guy 2 : Wow, your allways seshwining!
Guy1 : Where have you been, looks like you've been partying!
Guy 2 : Yeah, I've been seshwining tonight, I seshwined the shit out of that place!