Webster's English Dictionary

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rarefy

rar.efy also rar.i.fy \-*-.fi-\ vb [ME rarefien, rarifien, fr. MF rarefier, 
   modif. of L raref]acere, fr. rarus rare + facere to make - more at DO 1: to 
   make rare, thin, porous, or less dense  2: to make more spiritual, refined, 
   tenuous, or abstruse  : to become less dense