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.
"People say I walk around like I got a S on my chest"- 50 Cent
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..."
referring to a woman who has large enough breasts that aren't proportionate to her body mass and makes her to appear that she has an ass on her chest when wearing a push up bra.
Ex.: a woman who is under 5' tall, weighs under 100 lbs. and has a size D cup breasts or a woman who is over 6' tall and under 170 lbs. and has a size G cup breasts.
That little chick was fit, but with those tits; she had an ass on her chest!