A woman who has a butt that is unattractive to the male gender, because it is rigid and flat like a man's. Thus, the woman doesn't have a womanly-butt, but a "man ass."
"Check out that women at the pool. I wouldn't even want to see her naked because she has a man ass."
Hot man ass is ass that is hot and is owned particularly from a man. It is not gay, however, to think someone has hot man ass, as it is in fact a very straight thing to think.
Greg: Man, that guy has some hot man ass.
Kerry: That's gay, Greg.
Greg: No it's not! It's hot man ass!