Apparently because it's short for nipples, and Americans don't like to be reminded that they exist.

"Nips" is slang for nipples and was a racial slur for the Japanese once upon a time. I don't think the former is really gender specific, and I don't think most people have heard it used in the latter way.

Interesting, hadn't heard this yet.

As an aside, a (German) acquaintance of mine was once harshly told to shut it and kicked from an open source project's IRC channel because he had (by accident) used the German form of "yep" instead of saying "yep" or "yes" - it's "jap". Wondering to this day how someone can be offended by the mere existence of a word in a different language that happens to map to another one in your language - if it just happens to appear, without context. I'm stressing this as the discussion had nothing to do with people or getting out of hand.

I see thanks!