This slang term comes from @byeolbit who was showing me a tweet about VIXX that referred to Hyuk as "세젤 귀여운 혁"...
...but what does 세젤 mean?!
It turns out, its another "shortened" Korean word!
It combines these two words to make one:
세계 - World
제일 - First, most, best
Take the first syllable of 세계 and turn 제일 into 젤 and voila!
세젤 - World's most!