I recently shared the strange story of how Baltimore’s Friendship Airport (“BAL”) became BWI even though that code was already taken by Bewani, Papua New Guinea.
A lot of aviation codes have unique histories. Southwest Airlines couldn’t have SW — that belonged to Seaborn World at the time (it’s now Air Namibia). So they took WN which approximates Western and ultimately works nicely because it can be said to stand for We’re Nuts! Although sadly Southwest no longer serves peanuts.