/æz/
[adverb] Used to show that two things are equal, happen at the same time, or to give the reason for something. For example, 'She ran as fast as she could' or 'As it was raining, we stayed inside.'
'As' began as the Old English phrase 'all so', meaning 'entirely in this way' — a whole sentence squeezed into two letters.
The tiny word 'as' carries one of the most dramatic compression stories in the English language. It began life in Old English not as a single word at all, but as two: 'eall swā', meaning 'wholly so' or 'entirely in this manner'. 'Eall' meant 'all, entirely' — the same ancestor that gives us the modern word 'all' — while 'swā' meant 'so, thus, in that way', the same word that survives today as 'so'. Together they formed a phrase that expressed complete equivalence: 'in exactly this way, fully and without exception.'
Over the course of the Old English period, speakers began to run this two-word phrase together in rapid speech. 'Eall swā' became 'alswa', then 'alse', then 'als' in the various dialects of Middle English. The process is a perfect example of what linguists call 'elision' — the natural wearing-away of sounds that happens when a phrase is repeated thousands of times a day across many generations of speakers. By the time Chaucer was writing in the 14th century, the form 'as' was well established alongside the older 'als', and the shorter version steadily won out.
The word 'swā' — the 'so' half of the original phrase — is itself ancient, tracing back to the Proto-Germanic root *swa, which also produced the German 'so', the Dutch 'zo', and the Old Norse 'svá'. This means 'as' shares its deepest ancestry with one of the most common words in several Northern European languages. When a German speaker says 'so wie' (meaning 'just as'), they are using a near-identical construction to what an Anglo-Saxon speaker would have used over a thousand years ago.
What is remarkable about 'as' is the sheer range of grammatical work it does. It functions as a conjunction ('She sang as she worked'), a preposition ('He worked as a teacher'), a relative pronoun ('the same house as before'), and an adverb of degree ('as tall as a tree'). Few words in English carry such a varied workload. This versatility is itself a product of its history: because the original phrase 'eall swā' already expressed both degree ('to the full extent') and manner ('in this way'), the compressed form inherited both meanings and then stretched further still.
The related word 'also' preserves the older, fuller form, and looking at it you can almost see the ghost of 'as' inside it — 'al-so', all-so, 'eall swā'. The word 'so' itself split off from the same origin and went its own way. And the arch-casual filler use of 'as' in modern speech — 'He was, like, so as if...' or the teenage 'as if!' — shows the word still evolving, still taking on new shades of comparison and dismissal, still doing what it has always done: holding two things up beside each other and asking the listener to measure the distance between them.