Referring to the S on Superman's chest. When someone walks around like they got a s on their chest, they walk around like nothing can harm them or like they're invincible or something.
when a guy tries very hard to date the woman he loves or he feels attracted to but she inexorably turns down all his romantic proposals or she commits for a date but never turns up
"Hey Pal, how was the date with that smoking hot babe?"
"It was a disaster... she simply shit on my chest..."