Micheal Phelps's nickname?

Question

I've heard Micheal Phelps being referred to as the "Walrus". Does anyone know anything about it, is it true, where it came from, and how he received it?

Answer 1

i heard them call him "aquaman"

Answer 2

I thought it was "weishing young," meaning alien in Beijing. They mean it in a good way.

Answer 3

His nickname is The Baltimore bullet.

Answer 4

I heard his nickname is the Flying Fish