A place where castrated men hang out after being banished by their wives. Usually relegated to an out-of-the-way basement, while women take control of the main areas of the home. Said castrated "men" feel the need to add "man" to the word "cave" to otherwise impose masculinity when they clearly have none. For real men, the entire house is theirs, and there's no reason to add "man" to anything because it's implied that men own everything.
Denise banished Tom to the basement where he pissed and moaned with his friends about their wives. Out of spite, they dubbed their basement the "man cave" to feel in control of something.