[rar.e.fy] also rar.i.fy vb -fied ; -fy.ing [ME rarefien, rarifien, fr. MF rarefier, modif. of L rarefacere, fr. rarus rare + facere to make--more at do] vt (14c) 1: to make rare, thin, porous, or less dense: to expand without the addition of matter
2: to make more spiritual, refined, or abstruse ~ vi: to become less dense