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".)
"Whatever, dude just needs to get over his Manpain."
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.