Ace
A fighter pilot such as Baron Manfred von Richtofen - the 'Red Baron'.
Ammo
Ammunition
ANZAC
Australian and New Zealand Army Corps
ASC
Army Service Corps
BEF
British Expeditionary Force
Big Willie
The Kaiser
Billet
A house, barn or stable in which troops were stationed.
Bivvy
A bivouac, a makeshift shelter.
Blighty
Britain (often 'England'), that is - 'home'.
Boche
German