atonement n.

1590s, "be in harmony, agree, be in accordance," from adverbial phrase atonen (c. 1300) "in accord," literally "at one," a contraction of at and one. It retains the older pronunciation of one.

The meaning "make up (for errors or deficiencies)" is from 1660s; that of "make reparations" is from 1680s.

Atone. To bring at one, to reconcile, and thence to suffer the pains of whatever sacrifice is necessary to bring about a reconciliation. [Hensleigh Wedgwood, "A Dictionary of English Etymology," 1859]

The phrase perhaps is modeled on Latin adunare "unite," from ad "to, at" (see ad-) + unum "one." Related: Atoned; atoning.

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]