The correct term is lekku.

Darth is the title assumed by a Sith Lord or Master (one who is very powerful on the "dark side"). Wikipedia tells me that "it was once believed to be a condensed version of 'Dark Lord of the Sith,'" but Wookiepedia goes into more detail, which you can peruse at your leisure:

http://starwars.wikia.com/wiki/Darth

I'm pretty sure though if you stick with Darth = Evil, you'll be ok.

And there actually are female "Darths," even though we don't necessarily see them in the movies. Check out this one:


Darth Talon. If you can't tell from looking, she's pretty badass. And those things coming off of her head are not "tentacles" as I've always referred to them. They are called lekku, and all Twi'leks have them. More on that later.