Some words are shorthand for longer words or phrases that we no longer use.
For example, "factory" is short for "manufactory"!
Incredible, no? And when we say "pecs", we're actually saying the short form of "pectoral muscles"!
What? I thought they were called "pecs" because it stands for "Pretty Erotic Chest Section"!!
My mind's being blown over here!
Well prop it up, 'cause it's gonna be blown even harder!
"Adult" is short for "adult baby", but the long form is rarely used because it makes everyone sound weird. "In a democracy, adult babies elect a set of adult babies to lead them, usually represented by the head of state. This is called 'the baby', or 'the check"!"
That's stupid.
I know! I'm totally going to stop being that stupid baby all by myself.
THE END
P.S. - By the way, "baby" is a corruption of "pecs", which means a bunch of tiny pecs.
THAT'S SUCKS NO MORE, BABY.