h₂éḱ-

The root *h₂eḱ- ("sharp, pointed") generates a semantic field encompassing physical sharpness, sensory acuity, and metaphorical keenness across the IE family. Latin ācer ("sharp, keen, fierce") yields acrid, acerbic, acumen ("sharpness of mind"), acute (acūtus, "sharpened"), and exacerbate. The noun acus ("needle") gives acupuncture. The derivative acid (acidus, "sour," from the sharp taste) and acetic (from acētum, "vinegar") extend the root to gustatory sharpness. The noun aciēs ("edge, battle line" — the sharp edge of an army) gives the military vocabulary of armies drawn up for battle. Greek akmḗ (ἀκμή, "point, peak, prime") and ákros (ἄκρος, "topmost, outermost") yield acme, acrobat ("walking on points," i.e., on tiptoe), and acropolis ("highest city," the citadel). The medical term oxy- (from oxýs, "sharp, acid," a derivative of *h₂eḱ-) gives oxygen ("acid-born"), oxymoron ("sharp-dull"), and paroxysm. Sanskrit áśri ("edge, corner") and the related áśman ("stone," the sharp material) show the satem development *ḱ > ś. In Germanic, Old English ecg (Modern English edge), Old Norse egg ("edge"), and the compound Old English ecg-plega ("sword-play," i.e., battle) continue the root. The phonological development *h₂eḱ- > *ak- > Germanic *ag- > edge shows Grimm's Law and Verner's Law in sequence. The metaphorical extension from physical sharpness to intellectual keenness (acumen, acute, acerbic) and gustatory sharpness (acid, acrid) occurs independently in multiple branches, suggesting that the metaphor was already productive in PIE.