You have it slightly wrong - what is being said is that "You've got (a) real knack for this"= you've got real talent for this. "Knack" is pronounced "nack". It sounds very close to "neck".

There is a British idiom: "to have a brass neck" which means to be very audacious or daring, with no sense of shame or embarrassment. It is frequently shortened to just "having a neck". "He has a brass neck, asking for a promotion - everyone knows he is lazy and incompetent." "You've got a neck - coming to my birthday party after you have been rude and offensive to me all year!" That guy has a neck! He has just dumped his girlfriend, and now he is asking her sister for a date!"