'atoneness' originates from English, specifically formed from the verb 'atone' plus the suffix '-ness', where 'atone' itself comes from the phrase 'at one' (from the elements 'at' + 'one') meaning 'in unity or agreement'.

'at one' (an English phrase) developed into the verb 'to atone' in Early Modern English (c.16th century), and later the noun-forming suffix '-ness' was attached to create the modern noun 'atoneness'.

Initially the root phrase meant 'being at one, in agreement'; over time the verb 'to atone' gained the sense 'to make amends or reconcile for wrongdoing', and 'atoneness' now carries both the sense of 'state of reconciliation/atonement' and the sense of 'unity or oneness'.