Divide it:
こいつ=this guy (I might say a stupid thing, but I suppose it is a short for この奴)
マジ=really, seriously....
きめ=wood shaving. I guess it is a slang meaning? I had found it in the past in a book and on a dictionary it said used for a small low cheat person (I thought it could have worked to translate what young nowadays call "losers")...maybe gross is more the translation or maybe used for both. I just go with Sashimister translation coz it is surely right
