late 14c., ajusten, "to correct, remedy;" reborrowed by c.1600 in sense "arrange, settle, compose," from Middle French adjuster, Old French ajouter "to join" (12c.), from Late Latin adjuxtare "to bring near," from Latin ad- "to" (see ad-) + juxta "next," related to jungere "to join" (see jugular).
Influenced by folk etymology derivation from Latin iustus "just, equitable, fair." Meaning "to arrange (something) so as to conform with (a standard or another thing)" is from 1660s. Insurance sense is from 1755. Meaning "to get used to" first recorded 1924. Related: Adjusted; adjusting.