1. hirtus — de Vaan
hirtus 'hairy, shaggy' [adj. o] (Lucil.+) Derivatives: hirsutus 'hairy, rough* (Cic.+). Pit. *xe/irk-to-, *xe/irk-so- 'rough-haired'? These forms may represent *herto-, with dialectal raising of *e in front of rC. They are connected with horreo *to be stiff, which is semantically in order; but since the PIE root was *ghers-, it is unclear how -s- could be lost from a preform *ghers-to- > *hersto-. Alternatively, … — [de Vaan, s.v. hirtus, p. 300]