It’s the nickname of Usopp meaning “King of Snipers” in ONE PIECE.
“So” of the first character means “Aim”, “Geki” of the second means “Shoot”, and “Ou” of the last means “King”.
Other Japanese Manga characters good at shooting are Daisuke Jigen in “Lupin the Third”, Nobita Nobi in “DORAEMON”, etc.
What is your favorite word in “ONE PIECE”? I’m waiting for a request.