The story
Probably coined during World War I when the disease was common among soldiers in the trenches during trench warfare.
Etymology from Wiktionary, CC BY-SA 4.0.
trench mouth
Today it means: Acute necrotizing ulcerative gingivitis, a severe bacterial infection of the gums, typically characterized by inflammation, bleeding, deep ulceration, necrotized tissue, pain, fever, enlarged lymph nodes, fatigue, and halitosis. Full definition →