Grande Armée slang is a type of language that was used by soldiers in the French army during the Napoleonic Wars. It was like a secret code that only the soldiers knew so that they could talk to each other without the enemy understanding what they were saying.
The language was made up of different words and phrases that had special meanings. For example, instead of saying "I'm hungry," a soldier would say "I'm on the square." The word "square" meant "stomach," so the soldier was really saying that he had a hungry stomach.
Grande Armée slang was also used to make fun of officers or to talk about things that were forbidden. Soldiers would use words that sounded similar to official words, but were actually something else. For example, they would say "coward" instead of "colonel" to make fun of the officers who were afraid of battle.
Learning Grande Armée slang was like being part of a secret club. Soldiers who knew the language could communicate with each other without anyone else understanding what they were saying. It was a way for them to bond and feel like they were part of something special.