eager

Eager comes from Middle English egre, borrowed from Old French aigre, egre, which traces back to Latin ācrus (a variant of ācer, "sharp, keen"), from Proto-Italic akris and ultimately Proto-Indo-European h₂eḱrós, built on the root h₂eḱ-.

The word started out describing physical sharpness or sourness — the bite of something acrid — before it settled into describing a sharp, keen state of mind: enthusiasm.

It shares its Latin root ācer with vinegar and alegar, words that preserve the older, literal sense of sourness or sharp taste.