WN is our IATA airline code like AA for American and DL for Delta. When we came into business SA was already spoken for by South African Airways, and SW was being used also. Consequently WN was an arbitrary assignment like
FL belongs to AirTran. These codes are used for almost everything. In master schedules like the Official Airline Guide, they signify the operating airline. The FAA uses them to identify specific flights on air traffic control computers. They are part of SITA (formerly teletype) addresses. A lot of our aircraft use WN as the suffix in the registration.

(IATA is International Air Tranport Association)