An Etymological Dictionary of Astronomy and Astrophysics

1. To make less dense.
   
2. To become less dense; become thinned.
   Related concepts: → attenuate, → dilute, → decompress.

Etymology (EN): M.E. rarefien, from M.Fr. rarefier, from rare, combining form of rarus “loose, wide apart, thin, infrequent.”

Etymology (PE): Alaridan, infinitive of âlar, from intensive/nuance prefix â- + lar “thin, meagre” (Dehxodâ), Lori, Laki larr, larrek “thin cow or sheep,” Kurd. lerr “thin, lean,” variants of laqar (Torbat-Heydariyé), lâqar “lean, meagre, slender; weak.”