Mistaken means wrong about something. You are mistaken, a mistaken belief, mistaken identity.
It is gentler than wrong because it locates the error in the belief rather than the person — telling somebody they are mistaken leaves them room. Mistaken identity is the standard phrase for being taken for somebody else, and it is a real and serious problem in eyewitness evidence.
1adjectivemistakenWrong about something.
“I was mistaken about the time of the meeting and arrived an hour early.”
mistaken is a quality of error.
Mistake plus -n, from Old Norse mistaka, to take in error — mis- plus take.
Old Norse
mistaka — to take in error, to miscarry
Middle English
mistaken
English
mistake
English, today
mistaken
Morphologically mistake + -n. mistake comes from Middle English mistaken, from Old Norse mistaka (“to take in error, to miscarry”); equivalent to mis + take. Cognate with Icelandic mistaka (“to mistake”), Swedish missta (“to mistake”) (before apocope misstaga). The noun, which replaced earlier mistaking, is derived from the verb. Compare Swedish misstag (“mistake”), Faroese mistak (“mitake”), Icelandic mistak (“mistake”).
Etymology from Wiktionary, CC BY-SA