A combination of two seldom used expressions of awe, namely, 'super' and 'mega'. The two words join to form twice the awe, thus making this word, by definition, awe'some'.
(note: the word is often accompanied by a knowing look of smugness)
"Yo that guy's dancing style is totally fucking sega"
(said quietly),"seeeeeeeeegaaaa", by way of agreeing with something someone has said.