LOL. I wondered the same thing. I won’t tell you how old I was before I finally figured out what it meant. ;)
Slow learner, like me, eh? Even knowing what it means made it somewhat disturbing, since my mother put it on my bedroom wall instead of my brother's. You know how kids are, they always think one of their siblings is more favored? My mother slipped & confirmed it. Course, I gave her plenty to suffer me for, so can't say that I blame her. LOL