The [[stand (jojo)|stand]] possessed by [[Magenta Magenta]] in Steel Ball Run,  Part 7 of the [[Jojo no Kimyou na Bouken]]. 
It is a defensive stand that Magenta Magenta wears around his body to redirect all damage away from himself.

Namesake is likely "20th Century Boy", a song by the British band T. Rex, released as a single in 1973 with "Free Angel" as the B-side.
20th century boy (stand)