ok darkside sounds like a damn line term.
there can be no distinction between frontside and backside on skis, we all face the same straight direction going down the hill, so we are either spinning left or right. either way is the exact same as the other way, just a different direction. on a snowboard, since you face either left or right, depending on the way you spin (on a 180 for example) your 'front side' of you will face the landing, or if you spin the other way your 'back side' will be facing the landing.
in terms of rails (snowboarding), if you come in, say left foot forward, and the rail is to the left side of you and you do a boardslide facing down the rail, then it is a 'backside boardslide' because you approached the rail with it on your 'back side' this comes from skateboarding and can be very confusing.
for skiers, we mostly hit rails straight on, but say you come from the left side of a rail and ollie up and slide it right foot forward, then you are doing a lip slide because you have to get your tails up over the rail in order to slide it. so either sliding a rail normal or lipsliding it are the only two distinctions (aside from regular and unnaural)
and as for spinning off a jump, you can spin your 'normal' way, which is usually determined (but not always) by which foot is dominant, but based on which way is most 'natural' for you to spin. the other direction would then obviously be your 'unnatural' way of spinning. this applies exactly the same for taking off switch.
the word dirty came into play when tanner and boyd and a few of us just started saying it because (boyd mostly) we thought it was a funnier and better way of saying the word 'unnatural' of course the boys at freeskier are going to try to pick up on the lingo of the influental riders and use it in their mag, it's one of the ways our sport progresses, of course we just hope they will use the terms the right way, which isn't the case all the time for everything.
blah, enough of my rambling... call it what you want, we all pretty much get the picture, although i think the word fakie just sounds kinda gay in my opinion