/saʊnd/
[noun] A vibration that travels through the air and can be heard by the ear. The word can also be used as a verb meaning to make a noise, or as an adjective meaning healthy, strong, or reliable.
'Sound' carries four completely unrelated meanings that arrived in English from four different languages over a thousand years.
Few words in English are as secretly crowded as 'sound.' What looks like a single, simple word is actually four different words that collided in English over roughly a thousand years, each arriving from a different place, each meaning something entirely different. This makes 'sound' one of the most remarkable cases of what linguists call homonymy — words that look and sound alike but have completely separate histories.
The oldest of the four is 'sound' meaning a narrow body of water, as in Puget Sound. This comes from Old Norse 'sund', meaning a swimming or a strait between two landmasses. Old Norse sailors and settlers brought it into Old English during the Viking Age, roughly the 9th and 10th centuries. It is closely related to the Old English verb 'swimman' (to swim) and shares roots with the Proto-Germanic word for swimming. This geographical 'sound' is still alive in place names along the coasts of Scandinavia, Britain, Canada, and the United States.
The 'sound' that means healthy or whole — as in 'sound advice' or 'a sound sleep' — has Old English roots as well. It descends from Old English 'gesund', meaning healthy or uninjured, which itself traces back to Proto-Germanic 'gasundaz'. This is the same root that gave German 'gesund' (healthy) and Dutch 'gezond'. It is also present in the English farewell 'goodbye' — an old compressed form of 'God be with you' — and, fascinatingly, in the toast 'to your health.' This sense of 'sound' meaning solid, reliable, or mentally healthy has been in continuous use since before the Norman Conquest of 1066.
The 'sound' that means noise or auditory sensation is the one most people think of first. This word entered Middle English in the 13th century from Old French 'son', which came from Latin 'sonus', meaning noise or tone. Latin 'sonus' derives from the Proto-Indo-European root '*swene-', which referred to making noise. From this same Latin root we get a rich family of English words: Latin 'sonus' → Old French 'son' → Middle English 'soun' → English 'sound'; and also Latin 'sonus' → English 'sonic', 'resonance', 'sonata', 'sonnet' (via Italian 'sonetto', a little sound or song), 'consonant', and 'dissonance.' The final letter 'd' was added to the English word in the 15th century by analogy with other words, not because it reflected any change in pronunciation.
The fourth 'sound' — the verb meaning to measure the depth of water by dropping a weighted line — comes from Old French 'sonder', meaning to probe or test the depth. This Old French word is related to 'sonde', a plumb line or probe. Despite a long-standing temptation to connect this word to the acoustic sense of sound (after all, modern submarines use sonar to measure depth with sound waves), the etymology is entirely separate. It meant physical probing long before it could have meant acoustic measuring. Today we use this sense metaphorically when we say we are 'sounding out' someone's opinions — probing their feelings carefully, just as a sailor once probed the ocean floor.
What makes 'sound' so extraordinary is not just that four words merged into one spelling, but that English speakers seamlessly handle all four meanings every day without any confusion. Context does the heavy work that history cannot. When you say something 'sounds right,' you are reaching into a Latin tradition of music and resonance. When you call a decision 'sound,' you are echoing Old English ideas of physical health and wholeness. Language, it turns out, is its own kind of deep water — full of hidden layers that reward those willing to sound them out.