We spent some time discussing the English word "atonement" itself. Some of us have tended to use it expansively, to refer to the Christ event, the whole reality of salvation offered to the world; for others--some critics or deniers, as well as advocates--it seems to have narrowed in meaning to refer to how human sins are expiated, or even penal substitution.
Words, of course, are slippery things. There is no ultimate solution in a resort to etymology or to "authoritative" definitions, but consideration of origins and of the ways words have been used can still be enlightening.
Much theological language in English has been influenced by biblical translation and the King James Bible in particular, although that version carries over much of the language of earlier Bibles. A number of these include the language of "atonement" to refer particularly to sacrificial rituals of the Hebrew Bible. But it's widely acknowledged that the word means "at-one-ment", or reconciliation.
The word has sometimes been attributed as a coinage to William Tyndale, who used it in his rendering of 2 Cor 5:18 to the effect that God "hath given unto us the office to preach the atonement"; modern translations tend to refer to this as a "ministry of reconciliation" but the sense of what atonement means here is clear, and not expiatory.
But in fact there are older uses than Tyndale, and remarkably the equal oldest instances are from no other work than Thomas More's History of Richard III; you can see them here. The sense there is certainly of reconciliation, and this is also what all the references in Shakespeare a century later are about. Richard III scores again in one of the bard's two uses of the term:
...he desires to make atonement
Betwixt the Duke of Gloucester and your brothers,
And betwixt them and my lord chamberlain;
And sent to warn them to his royal presence. (Richard III, Act 1, Scene 3)
All Shakespeare's uses are like this; "atone" always has two objects, direct and indirect, who are people; a person has to be "atoned with" another.
The use of "atonement" to refer to sacrificial offerings starts I think (h/t to Michael Jensen) with Coverdale, and continues into the Geneva Bible of 1560, where Exodus 29:33 is "And they shall eat those things wherewith the atonement was made, to consecrate [and] to sanctify them: but a stranger shall not eat [thereof], because they [are] holy.". KJV would go on to render this and comparable texts similarly.
This is an intriguing interpretive move, namely to render the Hebrew words related to expiation (from the root kpr) in terms of reconciliation.
The subsequent history of the word is complex, but it is probably fair to say that despite the use of "atonement" in English Bibles to render these words related to sacrificial expiation, "expiation" itself as an idea has won out by subverting what "atonement" means; for despite Thomas More and Shakespeare's uses about him, few people who might have turned their minds recently to Richard III's need for atonement are imagining his reconciliation with enemies; instead we think of the wicked monarch's need for purging from sin.
Perhaps this is closer to at least some of the original biblical language, including not just the Hebrew of the OT but the metaphorical uses of cultic language in the NT in relation to how Jesus' death and resurrection effect salvation; but still, there is a bold and profound statement about the deeper truth that lies underneath cultic, economic and other metaphors that struggle to render how God deals with the world, in the original meaning of "atonement" as reconciliation. Tyndale perhaps deserves this credit, unless Paul does; for in the same verses of his NT, where the first English use of "atonement" in the Bible appeared, we read more fully:
For God was in Christ, and made agreement between the world and him self, and imputed not their sins unto them: and hath committed to us the preaching of the atonement (2 Cor 5:19, Tyndale)