late 14c., ajusten, "to correct, remedy," from Old French ajuster, ajoster "add; assemble; calibrate, gauge, regulate," from Late Latin adiuxtare "to bring near," from ad "to" (see ad-) + Latin iuxta "next, close by," from suffixed form of PIE root *yeug- "to join."
In 16c. French corrected to adjuster, but the pedantic effort was rejected and Modern French has ajouter. Influenced in form and sense by folk-etymology, as if from ad- + iustus "just, equitable, fair." English reborrowed the word by c. 1600 in sense "arrange, settle, compose," from French adjuster "fit (things together) properly, put things in order." Meaning "to arrange (something) so as to conform with (a standard or another thing)" is from 1660s. Insurance sense is from 1755 (see adjuster). To adjust to "get used to" is attested by 1924. Related: Adjusted; adjusting.