atone: [16] As its spelling suggests, but its pronunciation disguises, atone comes from the phrase at one ‘united, in harmony’, lexicalized as atone in early modern English. It may have been modelled on Latin adūnāre ‘unite’, which was similarly compounded from ad ‘to, at’ and ūnum ‘one’. => at, one
atone (v.)
1550s, 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 phrase perhaps is modeled on Latin adunare "unite," from ad- "to, at" (see ad-) + unum "one." Related: Atoned; atoning.