I can't get past the verb "pant" in panties. I vote to keep and cherish this word. And ladies, I suggest you whisper it unexpectedly in a slow and sultry voice to your special someone and see what reaction you get! Conversely, what man in his right mind would whisper "manties" to anyone? I'd choke with laughter if I witnessed that.
I've always thought "panties" was a very cute alternative to "underwear" (along the same lines as "footsies" or "toesies"). One of my youth group girls introduced me to "chonies" recently. Gah! Sounds like an organ.
Underpants is an absolute hoot. That word was made to be laughed at. Skivvies is a great word, too, but it applies to many different types of undergarment. I would think you'd rather call them manties, if only for the sake of precision.
But... see, rt? This is exactly what I was talking about. *eville grin*
Underpants is indeed far more hilarious than... well, almost any other word. However, uttering the word manties makes any male in the vicinity have an allergic reaction that is most pleasing to behold.