Etymology: Portmanteau of shame and anus.
1. a: Judging the anus to be obscene b: the susceptibility to such feelings.
It came about as a result of the anus being photoshopped out of many images of female nudes across the internet, and the lasting psychological damage it caused on the women thus depicted.
Brian: "Yea man lets tap that"
Brian: "Darn, I wanted to do her in the butt first!"
Andrew: "Sorry man, maybe next time"