I have a nknack for (something)
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I'm good at (something)
NO! NO! NO!
Your original idiom quote is COMPLETELY incorrect!!!
Either you yourself misheard it it, or you read what someone else wrote, who misheard it and wrote it down incorrectly!!!
The word in this case is NOT "neck" (which is the part of the body which connects the head to the shoulders!) but rather, the expression is " to have a knack for something". (The "k" is silent"; the vowel / the word rhymes with "back"!) The etymology of the word is uncertain - it originally meant a trick; but now it has no negative connotations: The explanation of the meaning of the phrase is otherwise completely correct: to be good at something; to have a natural talent or facility or ability to do something!