Rarefied was formed from the verb rarefy, which derives from a combination of the Latin rarus ("thin" or "rare") with facere ("to make") and has meant "to make thin" since the 14th century. In its original uses back in the 1500s, the adjective rarefied was on the lean side too; it meant "made less dense" (as in "the fog lifted and we could breathe more easily in the rarefied air"). By the 17th century, rarefy had gained the sense "to refine or purify," and over time rarefied followed suit.