A mixture of the words "jackass" and "dicknose" . It is used as a derogatory term, and/or to hurt someone's feelings. Once called a jacknose, you should feel like an embarrassment to society...but you can giggle away at the sound of the word.
Marty- What the fuck are you doing you jacknose?
Russ- Sorry, I was just counting pencils.
Marty- Yes, I am wearing a sweater vest.
Russ- Yeah, you look like an absolute jaknose...