Lufthansa is Germany’s national airline, and is named after “luft”, the German word for air, and the Hanseatic League, an old German trading bloc.
The United name originated in 1929 when William Boeing, yes that Boeing, merged his company with Pratt & Whitney and began buying up airlines across the US.
Delta Air Lines started life in 1923, with began passenger services from around 1928. It is named after the Mississippi delta where it was once based.
Rainman will happily tell you that the airline Qantas is an abbreviation for Queensland And Northern Territory Aerial Services. Header image: Airbus over Henson Park by Naparazzi on Flickr