h₂ḱew-

Root for hearing/perceiving, yielding Greek akouein (to hear), Latin cavere (to beware).‍​‌​‍​‌​‌​‍​‍​‌​‌​‍​‍​‌​‍​‌​‌​‌​‍​‌​‌​‍​‌​‍​‌​‍​‌​‍​‌​‌​‍​‌​‍​‌

Discussion

The PIE noun *h₁éḱwos (horse) is one of the most culturally and archaeologically significant reconstructions in comparative linguistics.‍​‍​‍​‍​‍​‍​‌​‍​‌​‍​‌​‌​‍​‍​‍​‍​‍​‍​‍​‌​‌​‍​‍​‌​‍​‌​‌​‍​‌​‌​‍​‍ The horse was central to PIE culture — for transport, warfare, ritual, and eventually the chariot technology that transformed Bronze Age civilisation — and the word's survival across the major branches reflects this centrality.

Latin equus (horse) continues the form with regular Italic phonology, generating the English learned vocabulary: equine (pertaining to horses), equestrian (a horse rider), and the rarely used equid (a member of the horse family). The word equity (from aequus, "level, equal") is NOT from this root despite the phonetic similarity — a common confusion.

Greek híppos (ἵππος, "horse") shows the characteristic Greek treatment of the PIE labiovelar *ḱw as a labial (h from the laryngeal + pp from the geminated labial). The Greek form is far more productive in English than the Latin one: hippodrome (horse-running course), hippopotamus (river horse — híppos + potamós), Philip (phílos + híppos, "horse-lover"), and the medical term hippocampus (a seahorse-shaped brain structure).

Sanskrit áśva- (horse) preserves the root with the expected satem treatment: PIE *ḱ > Sanskrit ś. The word appears frequently in Vedic texts and in compound proper names: Aśvins (the twin horsemen of Vedic mythology), and the Ashvamedha (the Vedic horse sacrifice, one of the most important royal rituals in ancient India). Old Persian asa- and Avestan aspa- continue the Iranian reflexes — visible in the personal name Astyages and place names throughout the Iranian world.

Old Irish ech (horse) and Gaulish epo- (as in the divine name Epona, goddess of horses) provide Celtic attestation. The pan-Celtic veneration of Epona — whose cult spread throughout the Roman cavalry — confirms the horse's sacred status in Celtic societies.

The Tocharian forms (A yuk, B yakwe) extend the attestation to the easternmost IE branch, confirming the word's PIE status.

Notably absent from the regular cognate set is the Germanic branch. English horse (OE hors) and German Ross are from different roots entirely — the PIE horse-word was lost in Germanic and replaced by vocabulary of uncertain origin. This replacement is one of the unsolved puzzles of Germanic lexicology: why did the branch most associated with horsemanship in the early medieval period abandon the inherited PIE horse-word? The answer may lie in taboo replacement, substrate influence, or simply the accidents of lexical competition.

Notes

Pokorny 587-588. English acoustic from Greek.