It is the nickname of Luffy, the protagonist of the One Piece Anime. Red hair shanks gave him a straw hat which he wears all the time that's why he is known as the Straw hat
Trafalgar Law is known as the Surgeon Of Death because of his surgical skills. He is one of the most Powerful Pirates of One Piece.
For his previous school volleyball team, Tobio Kageyama was known as "The King of the Court" because he always ruled the court. He ordered his teammates what to do or what not to do.
Hinata is short in height but still wants to play volleyball. Everyone underestimates him in the court but when he jumps higher than anyone else, everyone's eyes are on him.
Ryomen Sukuna is the king of curses. His (20 fingers sukuna) power level is almost equal to Gojo's power. He is quite aware of his power level and is arrogant about it.
Kakashi Hatake is also known as copy ninja because he can perform thousands of jutsus copied by his Sharingan.
Gintoki is known as White Demon because his powerful sword skills, range of abilities, silver hair, and white coat that he wore in bottles made him look like a demon.