what こいつマジきめ means?
 

really have no idea

"This guy's really gross."

if you were told that.... this is clearly a bad sign o.o

My Japanese must be pretty bad - it looked like it said 'koi majiki me' to me or 'koitsu majiki me'. . . which I can't see as to meaning what you just said :S

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 ;)

I'm going to go out on a limb and guess that きめ comes from 気持ち悪い>> きもい＞＞きめー。　I think the jump from きもい to きめ is the same as 怖い to こえー or 面白い　to おもしれー. Correct me if I'm wrong, but that makes the most sense considering someone's being called 'gross'.

Ah! Thanks for explaining that :) It actually helped a lot, mainly because I just realised I was mistaking the 'ki' for Katakana, so instead of 'maji' and 'kime' I was reading it as 'majiki' and 'me'. Thanks though, I'll be sure to be more careful in reading next time and divide the sentence more carefully.

Wow that would be a big shortening. From 6 syllables down to 2. But with my poor knowledge I cannot confirm or deny, can just ride along a wait for a native to confirm :)

I'm not so sure about that. I think おいー＞おえぇ〜 in slang. So 怖い becomes こわえぇ, not こえ. Making きもい into きめ〜 doesn't feel like it's correct (well, not in the dialect of Japanese I speak), but as a non-native I could be wrong.

I just wanted to throw my one cent in until a native chimes in with his opinion.

On the topic of こわえ versus こえー, I think I said こえー because I remembered this video: YouTube - バナナマン　怖い話. About 30 seconds in it sounds like they say こえー multiple times. I agree though, きもい to きめー does feel weird, it just made the most sense to me in this case. Anyways, like everyone's said, gonna defer to a native on this.