A boy or man who is particularly close with his mother to the point of being overly dependent on her. Lynn dumped Mark because he was such a mama's boy and always ran to her to fix his problems.
A sissy, especially a boy or man excessively attached to his mother. For example, The children called Tom a mama's boy because he ran home with every little problem. This sexist expression has survived despite its pejorative tone. [Colloquial; mid-1800s]