M.O.—Medical Officer. A foxy cove who can't be fooled with faked symptoms.
Mess tin—A combination teapot, fry pan, and plate.
Military cross—An officer's decoration for bravery.
Military medal—A decoration for bravery given to enlisted men.
Mills—The most commonly used hand grenade.
Minnies—German trench mortar projectiles.
Napper—The head.
Night 'ops—A much hated practice manoeuvre done at night.
No Man's Land—The area between the trenches.
On your own—At liberty. Your time is your own.