From Middle French sonde (“sounding line”), from Old French sonde (“sounding line”), from Old English sund- (“sounding”), as in sundġierd (“sounding-rod”), sundlīne (“sounding-line, lead”), sundrāp (“sounding-rope, lead”), from sund (“ocean, sea”), from Proto-Germanic *sundą (“a swim, body of water, sound”), from Proto-Indo-European *swem(bh)- (“to be unsteady, swim”). Cognate with Old Norse sund (“swimming; strait, sound”). More at sound.