The quality or state of being errant; a deviation from what is true, right, or proper; a wandering or straying.
From errant (wandering/deviating) + the suffix '-cy' (quality of). This noun form developed in Middle English to describe moral and physical deviation.
Medieval literature is full of knights seeking to correct their 'errancy'—the word beautifully suggests that being lost morally and physically are the same problem requiring the same redemption.
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