Used by men to describe all women in an offhanded way when expressing their real opinions will cause them great troubles. For example they could be under threat of being slapped and dumped by their girlfriends, or being punched and kicked into a pile by their buddies.
Girlfriend: What is your impression of Nancy?
Boyfriend: (Long silence as he fights off the fantasy of being a drop of sweat that glided down her neck into her cleavage) Errrr, I think she's nice.
(Real opinion: I wish you have legs and nipples like hers.)
Peter: What do you think of my new girlfriend?
John: Hmmmm... I think she's nice.
(Real opinion 1: Oh god, she's smoking hot. I know who I'll be sleeping with when you go away for your next business trip.)
(Real opinion 2: She's such a loud bitch, how could you date a hag like this?)