It's a baby that looks like an old man. Or an
adolescent male who attempts to act like he is grown but in turn appears even younger than he is.
Google Image "
baby jesus baroque paintings" and you'll see
(ex. 1)
Ben: Did you see that baby
over there? Totally looks like an old man!
Jerry: That's a baby man child!
(ex. 2)
Amy: That freshman
over there thinks he's so cool, he's bragging about how many girls he got with last night.
Caleb: It makes him seem like he's still in
middle school, such a baby.
Cara: That makes him a baby man child!