"qua" is a preposition mostly used by lawyers and scholars. Many people think it sounds pretentious. It means "meaning" or "in the role of".
"If the president's father commits a crime, the president has conflicting responsibilities. Qua son, he should try to protect his father, but qua president, he should treat all citizens equally, and he should punish his father. "
"Tom stared at the animal. He had always lived in the city, so this was the first time he had ever seen a cow qua cow, not just qua beef."
People sometimes use "England" to mean "Britain". Since Scottish people live in Britain, but don't live in England, this makes them angry. If I said to a Scottish person, "What do you eat on special occasions in England?", he would reply "In 'England', qua Scotland, we eat haggis." Haggis is something Scottish people eat, but English people don't eat.