rarus

Uncertain. Potential cognates include Ancient Greek ἐρῆμος (erêmos, “lonely”), Lithuanian i̇̀rti (“to dissolve, fall into ruin”) and Proto-Slavic *oriti. *A root of the form *h₁reh₁- (“to separate”) is phonetically required by Greek ἐρῆμος (erêmos). However, Schrijver 1991 argues that *h₁reh₁- cannot regularly give rise to Latin rār-: according to Schrijver's sound changes, zero-grade Proto-Indo-European *h₁r̥h₁rós would give Latin *rarus with short -a- (compare ratus from *h₂rh₁tós), whereas e-grade Proto-Indo-European *h₁reh₁rós would give Latin *rērus with long -ē-. * Alternatively, from a root of the form *Hreh₂-. In this case, potentially cognate (with schwebeablaut) to i̇̀rti and *oriti. * De Vaan 2008 mentions a root *h₂erH- (“to disappear, perish”) as another possibility but dismisses it as unlikely.