Chief Keef. He explained it by saying "Chief Keef is from Tony Montana(Scarface Film), Sosa is Sammy Sosa(Baseball Player), Sosa is from when Gucci Man made Gucci Sosa(rap mixtape), you know I just like Sosa". So basically Chief Keef adopted the nickname according to the numerous individuals name Sosa which he can relate to. In the movie 'Scarface', Alex Sosa is a powerful drug lord who would later become Scarface's nemesis, Sammy Sosa is a great baseball player from the Chicago Cubs and Chief Keef is from Chicago which is often referenced to as Chiraq, Gucci Man made a mixtape titled 'Gucci Sosa', and Chief Keef looks up to Gucci Man and is currently co-signed to his record label '1017 Brick Squad'.
Chief Keef also had a song titled 'Love Sosa', as well as a mixtape titled 'Chief Sosa'.