One who is unable to keep up the pace.
Interchangeable with 'Getting your bag'
Origins: A half sesh known as Paul Griffiths.
Wake up and finish your drink you halfsesh
When one fails to finish, be it drinking, smoking, running, eating....etc etc. More generic term than 'light-weight'. Can be used in almost any situation where someone fails! Is also used when one vomits.
Come on Ant you halfsesh, cant believe you cant finish that drink. No NO! stop puking you f***ing HALFSESH!