Origins
The word 'just' occupies a remarkable position in English, functioning simultaneously as a moral term (a just ruler), a marker of precision (just right), a minimizer (just a minute), and a temporal adverb (I just arrived). These seemingly unrelated senses all flow from a single Latin root whose semantic core was the idea of exactness — of things being precisely as they should be.
'Just' entered Middle English in the fourteenth century from Old French 'juste,' meaning 'righteous,' 'fair,' or 'lawful.' The French word descended from Latin 'iūstus,' an adjective formed from the noun 'iūs' (right, law, justice). Latin 'iūs' is one of the most important legal terms in Western civilization, the foundation of 'jurist,' 'jury,' 'jurisdiction,' 'jurisprudence,' and the entire vocabulary of Roman law that underpins modern legal systems. It derives from the Proto-Indo-European root *h₂yew-, which carried the sense of a binding or sacred formula — law conceived as something ritually fixed and exact.
In English, the adjective 'just' appeared first, meaning 'morally upright' or 'conforming to what is right.' This sense remains fully alive: a just decision, a just cause, a just war. But beginning in the fifteenth century, speakers began extending the word into adverbial territory. The bridge was the idea of exactness. If a just measure was one that was exactly right — neither too much nor too little — then 'just' could mean 'exactly': 'just so,' 'just as I said,' 'just right.' From exactness came the sense of narrow margin: 'just barely,' 'just in time.' And from narrow margin came temporal immediacy: 'I just saw her' (a moment ago, barely in the past).
Modern Usage
The minimizing sense — 'just a friend,' 'it's just me,' 'just checking' — developed from the notion of 'nothing more than' or 'exactly and only.' When someone says 'it's just a scratch,' they invoke the precision of 'just' to draw a boundary: it is this much and no more. This use has become so pervasive in modern English that 'just' is now one of the most common discourse markers in the language, often functioning as a softener or hedge ('I just wanted to ask') with almost no semantic content at all.
The Latin noun 'iūs' produced an enormous family of English words through separate borrowings. 'Justice' came through Old French in the twelfth century. 'Justify' arrived in the fourteenth century, originally meaning 'to administer justice' before shifting to 'to prove right.' 'Adjust' (from Old French 'ajuster,' to make conformable to) contains the same root, as do 'injury' (Latin 'iniūria,' a wrong — literally something 'not right'), 'judge' (from Latin 'iūdicem,' one who declares the law), and 'prejudice' (pre-judgment).
A curious footnote: Latin had two completely unrelated words spelled 'iūs.' One meant 'law' and gave English its family of justice-words. The other meant 'broth' or 'sauce,' from a different PIE root, and survives in culinary English as 'jus' — the meat juices served 'au jus.' The coincidence is purely orthographic; the two words had different vowel quantities in classical Latin and were never confused by Roman speakers.
Middle English
The phonological development from Latin 'iūstus' to English 'just' is straightforward. Old French voiced the initial /i/ to /dʒ/ (as it did in 'jardin' from Latin 'hortus,' via intermediate forms), producing the characteristic 'j' sound. The Latin long vowel shortened in French and English, yielding the modern /ʌ/ vowel. The spelling has been stable since Middle English, though Chaucer sometimes wrote 'iuste' preserving the older French form.
In contemporary usage, 'just' is among the ten most frequently used adverbs in English. Studies of conversational English show it appearing with extraordinary frequency as a discourse particle — a use that would have been unrecognizable to the Roman jurists who first deployed 'iūstus' to describe the precision of righteous law.