When a male character's backstory related-angst breaks out, usually in isolation, brooding, douchebaggery, control or trust issues, misbehavior, crying a single tear, etc.
A term coined by fans who understandably got tired of seeing male characters' douchebaggery excused by a dark, tragic past, especially when the past involves hurting & dehumanize female characters solely to motivate the male hero.
Note: there is no female version of Manpain, because women's emotional responses to surviving trauma are always legitimate, no matter how extreme. (Anyone who denies this is just "blaming the victim".)
"The guy on that show was raped repeatedly as a child and saw his whole family killed by gangsters, that's why he has trust issues."
"Whatever, dude just needs to get over his Manpain."
When a grown man has the emotional life of an angsty teenager he is said to be experiencing manpain, especially if he tries to compensate with macho behavior.
There are many causes of manpain ranging from violently killed family member/mentor/lover(s) to a broken heart to absent parental figures or even a history of sexual and/or physical abuse.
Manpain is generally expressed in the following ways: breaking shit, drinking too much, picking fights in bars, becoming a costumed superhero and taking long drives while listening to wailing guitars. For the less violent it can be expressed by remaining stone-faced while flexing jaw muscles, staring broodingly into the middle distance and crying a single tear.
Blind with manpain, he drove into the desert and cried single tear while tracing the faces of his murdered family in a photograph.