Slut Talk

I'd like to address the issue of sluts.  I'm a "young woman" (19) and from the time I was in 7th grade to now, that word has gotten thrown a lot.  Sometimes it gets applied to someone who is not a slut, they just make other girls jealous.  And it is almost always applied to girls.  Never a guy.
So one of my questions is why do girls feel the need to judge other girls so harshly?  Why are we so competitive?  We feel the need to break other girls down by their actions or "wrong doing," but why?  There is ALWAYS going to be someone prettier than you, more fashionable, more everything.  Just accept that now and find some peace in it.
I also feel like everyone has a different definition of "slut," and that is another aspect of the frequent misuse of this word.  What's slutty to one person, might not be slutty to another.  To the Amish, shorts on girls (no matter the length) are probably thought of as whatever the Amish word for slut is.  Most american girls obviously don't feel that way.  Some girls embrace the word, on twitter they hashtag #teamslut and will openly admit it to anyone.  Yet this doesn't take away the fact that when called that in a fight or behind their back, it's hurtful and it usually pisses them off.
I honestly can't give my definition of a slut.  I guess it would be something like: a girl who openly degrades herself in public or elsewhere with a guy (ex: sex in public, not wearing pants, flashing people on purpose--not for beads).  But even that definition is more "slutty" than "slut."  I feel that one night you might drunkenly make a fool out of yourself, but that doesn't brand you a slut for life.  Does being a stripper make you a slut for life?  I don't feel that it does.
Would you call someone svelte?  No, because you don't know what it means (or probably how to say it), so WHY are we using the world slut?
So lets all put the word slut in our back pocket and use it sparingly or save it for jokes, instead of serious name calling.  At least until we have all agreed on a true meaning to the word.
Thank you.