"a making fit or conformable; the act of adapting to a given purpose; orderly regulation or arrangement," 1640s, from French ajustement (Old French ajostement) or else a native formation from adjust (v.) + -ment.
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.
common suffix of Latin origin forming nouns, originally from French and representing Latin -mentum, which was added to verb stems to make nouns indicating the result or product of the action of the verb or the means or instrument of the action. In Vulgar Latin and Old French it came to be used as a formative in nouns of action. French inserts an -e- between the verbal root and the suffix (as in commenc-e-ment from commenc-er; with verbs in ir, -i- is inserted instead (as in sent-i-ment from sentir).
Used with English verb stems from 16c. (for example amazement, betterment, merriment, the last of which also illustrates the habit of turning -y to -i- before this suffix).
The stems to which -ment is normally appended are those of verbs; freaks like oddment & funniment should not be made a precedent of; they are themselves due to misconception of merriment, which is not from the adjective, but from an obsolete verb merry to rejoice. [Fowler]