h₃nógʷʰ-

Root for nail/claw, yielding Latin unguis, Greek onyx, Old English naegl.‍​‍​‌​‌​‌​‍​‍​‍​‍​‌​‌​‍​‌​‌​‌​‍​‌​‌​‍​‍​‌​‍​‍​‌​‌​‍​‌​‍​‍​‌​‌​‌

Discussion

The PIE root *h₃nógʷʰ- (nail, claw) produced cognates in every major IE branch that have preserved the same dual meaning — fingernail and metal nail — for over five thousand years.‍​‍​‌​‌​‌​‍​‍​‍​‍​‌​‌​‍​‌​‌​‌​‍​‌​‌​‍​‍​‌​‍​‍​‌​‌​‍​‌​‍​‍​‌​‌​‌ The word is one of the most phonologically complex reconstructions in PIE, with an initial laryngeal, a nasal, a labiovelar, and an aspirate, yet the regularity of its reflexes across the family confirms the reconstruction with unusual confidence.

Latin unguis (nail, claw, hoof) and the related ungula (hoof, claw) continue the root with the expected Italic loss of the laryngeal and treatment of the labiovelar. The English derivatives are medical and zoological: ungulate (hoofed animal), ungual (pertaining to a nail or claw), and unguiform (claw-shaped). The Latin diminutive ungula became the standard anatomical and veterinary term for hooves.

Old English nægl (nail — both fingernail and metal fastener) preserves the dual meaning directly. The PGmc form *naglaz shows Grimm's Law operating on the labiovelar: PIE *gʷʰ > PGmc *g (with loss of the labial component). German Nagel, Dutch nagel, and Old Norse nagl continue the same form. The semantic identity of fingernail and metal nail is not an English peculiarity but a PIE inheritance: the metal fastener was named for its resemblance to a fingernail or claw — pointed, hard, and driven in.

Greek ónyx (ὄνυξ, genitive ónykhos, "nail, claw, hoof") gave English onyx (the gemstone, whose layered bands were thought to resemble a fingernail) and the combining form onycho- in medical terminology (onychomycosis, a nail fungal infection). The Greek form shows the loss of the initial laryngeal with compensatory vowel development.

Sanskrit nakhá- (nail, claw) and the related Avestan naxti- provide the Indo-Iranian reflexes. Lithuanian nãgas (nail, hoof) and Old Church Slavonic nogŭtĭ (nail) confirm the Balto-Slavic continuation.

The universal dual meaning of "nail" across IE — both the keratin growth and the metal fastener — implies that the semantic extension occurred in PIE itself, before the branches separated. This in turn suggests that PIE speakers worked with pointed metal (or bone) fasteners that they compared to claws. The word is thus indirect evidence for PIE material culture: people who hammered pointed objects into surfaces and named them after their own fingernails.

Notes

Pokorny 780. Latin un- from *h3n- with prothetic vowel.