The acronym stands for Defensively Equipped Merchant Ship(s). The Navy provided anti-aircraft and anti-submarine guns and crews to man them for a great many vessels in Canada’s Merchant Marine. Six Saskatchewanians perished while serving as part of such gun crews, see GA Burgess, GR Craig, A Leskiw, JE Michael, AR Stinchcombe and N Szuszwal.

http://en.wikipedia.org/wiki/Defensively_Equipped_Merchant_Ships