The opposite of a femboy, a masgirl is a masculine female and usually participates in crossdressing, and various other activities usually associated with men.
Any female who is more masculine than her boyfriend. Her physique is typically bulkier than that of her significant other. She also has a deeper voice and weighs more. It's speculated that mangirlfriends even have more facial hair than their boyfriends.
On top of all of this, a mangirlfriend is usually ugly as shit. If one didn't know she was a girl, one would swear she was a bear. Once one discovers she's a girl, one immediately assumes she's a lesbian.
But no, she is in fact straight, and usually extra slutty in hopes of proving this. A mangirlfriend's boyfriend is typically either desperate for companionship or is basically on the verge of being a twink. If the latter is the case, the difference in masculinity is extra humorous.
Mortimer: "What the hell? Why is Jacob sitting in his girlfriend's lap? Shouldn't it be the other way around?"
Jimi: "MANGIRLFRIEND!"
Kaylee: "Fuck Austin and his fucking mangirlfriend!"
Ajay: "Hey watch out, he might have her beat you up."
A woman, regardless of looks, who lacks almost all lady-like personalities, instead boasting male attitudes like fighting, disrespect for content women like (such as how a man would talk to a girl in an attempt to go out with her), etc.