http://www.f-16.net/articles_article10.html

The F-16 is often referred to as the "Viper", a nickname especially popular with people involved with the F-16. Before "Fighting Falcon" was selected as official name, pilots at Hill AFB, the first F-16 base, came up with a number of proposals, including "Viper". Lt. Col. Pat "Gums" McAdoo, USAF Ret., one of the first F-16 pilots at Hill AFB, recalls the origin of the name "F-16 Viper"