Origin and history of rarefy

[thin, few, unusual] late 14c., "thin, airy, porous" (opposed to dense); mid-15c., "few in number and widely separated, sparsely distributed, seldom found, very infrequent;" from Old French rer, rere "sparse" (14c.) and directly from Latin rarus "thinly sown, having a loose texture; not thick; having intervals between, full of empty spaces" (antonym of densus). Sometimes reconstructed to be from a PIE root *ere- "to separate; adjoin."

"Having the particles not close together," hence "few in number," hence, "unusual." Sense of "remarkable from uncommonness," especially "uncommonly good" is from late 15c. (Caxton). Related: Rareness. In chemistry, rare earth is from 1818.

Rare implies that only few of the kind exist : as, perfect diamonds are rare. Scarce properly implies a previous or usual condition of greater abundance. Rare means that there are much fewer of a kind to be found than may be found where scarce would apply. [Century Dictionary, 1895]

"act or process of making rare or expanding a body of matter (originally chiefly gases) so that a smaller number of particles occupy the same space," c. 1600, from French raréfaction or directly from Medieval Latin rarefactionem (nominative rarefactio), noun of action from past-participle stem of Latin rarefacere "make rare or thin" (see rarefy). Rarefication (1610s) also is used. Rarefactive is attested from early 15c.