Word Origins •
|The notion of ‘bringing in close proximity’ is present in adjust. The source was the obsolete French verb adjuster, from Old French ajoster ‘to approximate’, based on Latin ad– ‘to’ and juxta ‘near’, source of words such as joust (Middle English) originally to ‘bring near to join battle’ and juxtapose (mid 19th century) ‘place near’.
—Oxford Dictionary of Word Origins
Don‘t assume that adjust is formed by adding the prefix ad– to the word just; these two words should come from different roots, though, according to some sources, adjust might be influenced in form and sense by folk-etymology, as if from ad– + iustus ‘just, equitable, fair‘.
Word Formation Resource •
adjustable, adjustment, maladjusted, maladjustment, readjust, readjustment, unadjusted, well-adjusted