A grown adult that acts very immature on the internet, rarely in real life, like a child. These adults are very loud and arrogant, annoying or straight up immature and stupid.
You usually see these people on youtube, mainly kids content / click bait.
Guy 1: That clickbaiter is so lame!
Guy 2: IKR, such a manchild!
A Person who acts so Immature, (Specifically a grown up.) They cry like there is NO fuckin tomorrow as a method to get stuff their own way. (Like caillou, Except older.)
See that guycrying and fartin about not getting a lawnmower? That right there my friend, is a MANCHILD.