mistaken adj.

mid-14c., "to commit an offense;" late 14c., "to misunderstand, misinterpret, take in a wrong sense," from mis- (1) "badly, wrongly" + take (v.) or from a cognate Scandinavian source such as Old Norse mistaka "take in error, miscarry." Perhaps a blend of both words. The more literal sense of "take or choose erroneously" is from late 14c. Meaning "err in advice, opinion, or judgment" is from 1580s. Related: Mistook; mistaking.