In the simplest of definition, a man child is an immature guy who just refuses to grow up. This doesn't mean he necessarily lives at home with mom and dad ― although he probably should considering the way he functions in the real world ― but that he just doesn't have his life together.

a grown man who is very immature, therefore considered a man child.

A man by age but a child in mentality and actions. Usually an inability to get over the "frat boy" or "party boy" phase. Can't have fun without a beer; oftentimes brews his own beer. Still relate to women as sex objects (even in they're married) and are emotionally undeveloped (e.g., fight for their rights mentality even if there is nothing worth fighting; rarely takes responsibility for own failings). A general irritant to women and men that have matured.

A man child is a male who is over the age of 25, who still lives as though he is a juvenile. Many man-children are not gainfully employed, and survive off of the financial support of their enabling parents. Heavy video-game addiction to fantasy games such as World of Warcraft are key to the man-child.

A man child is an individual (usually male) who still hold traits of a child (adolescent). These traits include whining, pettiness, insecurity and not being able to take the blame.

A term originating from Claude Brown's 1965 memoir "Man-Child in the Promised Land" about growing up in the streets of 1940s and 50s Harlem, NYC, one of the earliest and most powerful accounts of ghetto life and its culture. A "Man-Child" originally referred to a boy or adolescent from the streets who was prematurely exposed to and knowledgeable about sex, violence and crime, incarceration, adult responsibility for younger siblings due to a dysfunctional home, and hustling to make a living.

A man with the character of a young child. Also could be a juvenile who is too young to be old, but too old to be young. An old soul trapped in the body of an adolescent. A man who is too old for babysitting, but desperately needs it.

plural: manchildren

A Man Child is a fully-grown male that acts like a baby. A Man Child usually whines, complains, and thinks everything is unfair. Man Children in the workplace can be recognized as one who is always complaining about the rules and/or shows too much emotion ex. Crying.

Predominantly in their mid-thirties to early forties and typically hide in their mom's basement playing video games and watching my little pony hentai.
They refuse to act their age and yet want the world to treat them as adults when it is most convenient for them. Most have the appearance of an overweight slug and lack even the most basic of social skills.
More often than not these sorry individuals aren't really autistic, but simply are lazy and were raised wrong.
This trait is often paired with being a weeaboo or a male feminist who not only has never had sex with a woman, but likely hasn't seen their dick in years due to their obesity and perpetual sitting posture.
Other key signs that you are dealing with a man-child are:
1.) Refusal to see that they are in the wrong.
2.) Hisses at the mere sight of sunlight.
3.) Making shitposts on 4chan and Reddit about sensitive topics.
4.) Extremely homophobic and racist.
5.) Owns at least seven fedoras.
6.) Hasn't bathed in months.
7.) Posts bad photoshops of them and their waifu on social media.
8.) When angered, will launch into a foaming rage and rant on the internet on how they're being oppressed and discriminated against.
9.) Has severe arthritis in one hand from jacking it so often.
10.) Creates Sonic OCs.