Some definitions of "manchild" I found:
I've always been referred to as a manchild under #1
1. man child
a grown man who is very immature, therefore considered a man child.
Dude you need to grow up and stop being such a man child
2. man child
A man that is so large and/or powerful that he makes other men around him look like children. Usually an athlete that dominates others on the field/court because of his size and strength.
Did you see that hit? Ray Lewis is a man child!
David Ortiz is a man child at the plate.
3. man child
Someone who hits puberty really early and is like 11 butt looks 15
"See that kid over there with the big muscles and armpit hair and a beard?"
"what a man child!"
4. man child
a child acting adult mlike or an adult acting child like
im a man child
5. man child
the term used when a baby is born, and is NOT a child but in fact a fully grown teenager, common among births where the couple were relations or step brother/sisters
"Axe man, dam hes Petes and his step sisters Man Child"