rarefied - Dictionary definition and meaning for word rarefied
rare(,,very less)+fied(..like field which means environment), hence environment where GAS is VERY LESS DENSE..
rarefied is a past tense of rarefy. And, rarefy = rare+fy(to make), i.e. to make something rare, or less dense... :)
rarefied meaning is to make refine or pure so rarefied-ra+refined= rarefined