Interestingly (well, I think so) the word probably only exists in “Modern British English” in the fixed phrase “demob happy”, “adj. chiefly Brit. deliriously excited or elated because of (impending) release from a job, assignment, posting, etc.” (OED online, draft additions September 2008), eg in this quote from a comment in The Guardian’s education section earlier this month:
May is a lousy time in the classroom - and the kids are ready for a break - and demob happy as they see the yr 11s and 13s leave
although you can still catch occasional references in a historical context to “demob suit”, “a suit issued to a soldier upon demobilization”.