1540s, "having a conscience;" 1580s, of actions, "consonant with right or duty;" 1640s, of persons, "governed by conscience." It and conscioned "appear to be popular formations from conscion, taken as a singular of conscien-ce" by misapprehension of the "s" sound as a plural inflection [OED]. See conscience. Related: Conscionably. Obsolete from early 18c. but fossilized in its negative, unconscionable.
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