I was on a call recently, and someone said ‘OK, now that we’re all in agreeance, …’
That made me wonder about the word, which I’ve seen and heard occasionally. So I checked out the Oxford English Dictionary (natch), and there it is.
It’s a synonym for agreement, in the sense of ‘concord of opinions’ (rather than ‘contract’), and its origin is Scottish, dating back at least to the early fifteenth century. Agreement is older, apparently by about a century, and means both the concord and the contract thing.
OED notes that agreeance may now be regarded as ‘non-standard’, and the examples of usage are mostly Scottish and American (which is to say, non-standard for an Oxford lexicographer).