/ðʌs/
[adverb] In this way, or as a result of what has just been said. It is used to show that something follows logically from what came before.
'Thus' is one of the oldest surviving words in English, almost completely unchanged for over a thousand years.
Few words in English have lived as long and changed as little as 'thus.' It descends directly from the Old English word 'þus,' which had exactly the same meaning — 'in this way' or 'as follows.' Old English was spoken in Britain from roughly the 5th century onwards, and 'þus' appears in some of the earliest written texts that survive, including the works of King Alfred the Great in the 9th century. The letter 'þ,' called thorn, was a common Old English character representing the 'th' sound, so the visual difference between 'þus' and 'thus' is smaller than it appears.
The word traces back even further, to the Proto-Germanic root 'þus' or 'þa,' a demonstrative particle that pointed to something near or already mentioned — essentially a way of saying 'like this' or 'in such a manner.' This root is shared across many ancient Germanic languages. Old Frisian, for example, had 'thus,' and Old Saxon had 'sus,' both meaning the same thing. These are sister languages to Old English, descended from the same ancestral tongue spoken by Germanic peoples across northern Europe before they separated into distinct groups.
Go back further still, and you reach Proto-Indo-European, the ancient ancestor language reconstructed by linguists from patterns across hundreds of related tongues. The root here is believed to be a demonstrative stem related to 'so' and 'this,' pointing to something already present in the conversation. This same demonstrative instinct ripples through many modern European languages. Sanskrit had 'tu' as a particle of similar function, and Latin had 'tum' and 'tunc' for 'then' and 'at that time,' all cousins of the same ancient pointing gesture frozen into speech.
What makes 'thus' so remarkable is how stubbornly it has resisted change. Most Old English words were either replaced by French or Latin words after the Norman Conquest of 1066, or they shifted dramatically in spelling and sound. Yet 'thus' survived almost untouched, passing through Middle English — where it appears in Chaucer's writing in the 14th century as 'thus' — and into Modern English wearing almost the same face it had a millennium ago. This persistence is a sign of how deeply embedded the word was in the basic machinery of the language, too useful and too familiar to be dislodged.
Over time, 'thus' acquired a slightly formal or literary flavour, partly because it was so common in religious texts, philosophical writing, and legal documents throughout the medieval period. The King James Bible of 1611 is thick with it — 'Thus saith the Lord' — cementing its association with authority and weighty pronouncements. Today it sits at an interesting crossroads: still perfectly correct and widely used in writing, but carrying just enough gravity that in casual speech, most people reach instead for 'so' or 'therefore.' The word has not changed; it is the world around it that has shifted, leaving 'thus' standing like an ancient oak in a modern garden.