Why are French soldiers called poilus?: Information Roundup
Poilu as a slang name for a French soldier originated during the First World War and was at first applied only to a private fighting in the front lines. It literally means "hairy" and is formed from French poil, which is derived from Latin pilus, "hair." The generally accepted theory is that the French soldiers in the trenches on the western front were called poilus because they were often unable to shave off their beards and to cut their hair. By 1916 poilu was a common name for any French private soldier and it became the French equivalent of the British Tommy Atkins and the American Yank. It is possible that poilu as applied to French soldiers had a broader significance than the literal one. In the past, French soldiers had been notable for their fierce mustaches. Balzac, the French novelist, had employed poilu in the sense of brave or intrepid.