at the end of the Second World War, returning servicemen underwent a 'demobilisation' process at various centres around the country. As part of this process of transitioning back into civilian life, they were issued with an outfit of civilian clothing. This included a new suit, which became known as a 'demob' (short for demobilisation) suit. Accordingly, the phrase has become synonymous with any suit that looks sufficiently mass-produced and out of style that no-one in their right mind would buy it themselves.
He goes to so few formal occassions, the last time he did he was wearing his demob suit.
by spudsey February 09, 2010