Awesome - really really really (reccuring) good.
Mage - Other name for a wizard - user of magic.
Therefor if something is awesomage, it is something so unearthly and powerful that it practically is a word of God.
Generally used by itself, because no other words in the English language (only Norsk comes close) are mighty enough.
A common fault in spelling is the writing of an 'e' in the word after the 'm'. These people are wrong! Go and take a shower and wash the shame off of youself. These people offend Odin.
(Do you like Black Metal?)
Guy 2: "Ja jeg liker det veldig godt!"
(Yes I really like it!)
Guy 1: "Awesomage"
Guy 2: "Awesomage"
Disclaimer: Min norsk er dårlig.
(My Norwegian is bad.)