means A person or device that adjusts something—an instrument, an engine, a radio—into correct pitch, performance, or frequency.

from From the verb "tune," which itself comes from "tone" — Middle English drew it from Old French "ton" and ultimately Latin "tonus," "a sound or pitch," borrowed in turn from Greek "tonos," literally "a stretching" (think of a string pulled taut until it sings). The "-er" is the plain English agent suffix, the one we slap on anything that does a job. So a "tuner" is, at root, "one who stretches things into the right tension" — which is more or less exactly what tuning a guitar string still feels like.