English treats sound as a single familiar spelling, yet that spelling packs several unrelated words. The sense most people meet first is what the ear registers: a noise, a spoken phone, a musical tone. Next to it sit an adjective meaning healthy or dependable, a geographic noun for a sea strait, and a nautical verb for measuring depth. They look identical on the page. They did not travel the same road.
Hearing: a Romance loan with a later -d
The auditory noun reached Middle English in the late thirteenth century as soun, borrowed from Anglo-French and Old French son (also spelled sun), itself from Latin sonus. The matching verb arrived a little earlier, in the early thirteenth century, as sounen, from Old French soner (modern French sonner) and, in some uses, straight from Latin sonare. Both Latin forms go back to a Proto-Indo-European root reconstructed as swen- or swenh₂-, “to make a noise.” Distant relatives include Sanskrit svanati (“it sounds”) and Old English swinn (“melody”), which show that the root was already old in the family even though this particular English word did not come down through Germanic.
The extra letter at the end is not Latin. Between about 1350 and 1550 English settled an unetymological -d onto soun, the same habit that thickened thunder, pound (the verb), spindle, and kindred. Neighboring languages did something similar after n, especially when l or r sat nearby. By the time printed English was common, sound had become the normal spelling for the thing heard, and later compounds followed that form: the resonant plate in an instrument (sound-board, fifteenth century), sound-wave (mid-nineteenth), sound effect (early film era), sound barrier (late 1930s).
Whole and healthy: a native adjective
The adjective is older in English and never needed French. Around 1200 it appears as sounde, continuing Old English gesund, “safe, uninjured, with the faculties intact.” The prefix ge- (the same family as German ge-) dropped away in the Middle English form. Behind it stands Proto-Germanic sunda- / gasundaz, from a Germanic root swen-to- meaning healthy or strong.
That cluster is easy to see on the map of northern Europe: Dutch gezond, German gesund (and the sneeze-reply Gesundheit), Danish and Swedish sund. Old English swīþ (“strong”) and Gothic swinþs sit nearby, as does German geschwind (“quick”). The German group has further ties in Indo-Iranian and Balto-Slavic, so the “healthy” word is a deep Germanic inheritance, not a medieval import.
The first English uses stay close to the body: not diseased, not damaged. From about 1300 the word pairs alliteratively with safe. Later the sense stretches. Mid-fifteenth century it can mean right or free from error; by the 1520s, loyal to accepted opinion; by the 1570s, of an argument, free of logical cracks. Financial solidity is attested around 1600. Sleep that is unbroken is sound from the 1540s. None of those later shades required a new etymology. They are the old “whole” sense applied to judgment, money, and rest.
Water you can swim, and a line you drop
A third sound is a narrow channel of water. It is recorded around 1300 as sounde, from Old Norse sund (“a strait; swimming”) or from the matching Old English sund, which already covered the act of swimming, a stretch of water one could swim across, and a strait. Both come from Proto-Germanic sundam-, a suffixed form of the same Germanic root that gives English swim. In Middle English the noun could still mean swimming itself, or the ability to swim. Place names keep the geographic sense: Plymouth Sound, and the Sound between Zealand and Scania (Danish Øresund).
The fourth history is the verb meaning to fathom water with a line and lead, attested in the mid-fourteenth century (already implied by sounding). It comes from Old French sonder, from sonde, “sounding line.” Most modern etymologists treat sonde as probably Germanic, the same watery sund that named the sea. An older guess derived it from an unrecorded Vulgar Latin subundare, “to plunge under the waves.” That Latin story is now widely set aside; Barnhart, among others, rejects it. The French verb, once in English, soon left the ship. From the 1570s it could mean to probe a person or a situation for what is hidden: you sound someone out.
The two water words are therefore neighbors, not twins of the noise word. One is native (or Norse) geography tied to swimming. The other is a French technical verb that likely borrowed the same Germanic sea-word as its tool name. They only look like the Latin loan because English later wrote all of them with -d.
What the spelling conceals
Cognates make the split obvious once you look past English. French son and sonner still match the auditory loan. German gesund still matches the health adjective. Swim still matches the strait. None of those foreign forms collapse into one word the way English spelling did.
The origin of the word sound, then, is not a single root with four meanings. It is four arrivals that English happened to spell alike, after a stray -d made the French noise-word look as native as gesund and sund. The takeaway is small and concrete: if you say a sound night’s sleep after a sound crossing of a sound, you have used three different inheritances in one sentence, and none of them is the Latin sonus your ear might expect.