Atone
Middle English (originally in the sense ‘make or become united or reconciled’, rare before the 16th century): from at one in early use; later by back-formation from atonement.
wiktionary
From atone(“reconciled”), from Middle English atone, attone, atoon(“agreed”, literally “ at one”), equivalent to at + one. Compare Latin adūnō(“I unite, make one”) for the similar formation. Regarding the different phonological development of atone and one, see the note in one.
etymonline
atone (v.)
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. 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.
