Shortening of the word nutsack.
Shalil: Bro, it's so damn hot outside. I think I've got swamp neck.
George: Goddamn, swamp nack man.
Nack is an adjective: It means to sell something cheap that has a higher worth because you need a bag.
He nacked off that bike for five bucks and then went straight to the dope man
The flirty conversation before making out with a stranger at the club/bar
Oi bro, George was just nacking and macking with some chick in the smokers
shattered, tired, knackered on an extreme level
‘I’m absolutely nik nacked today, I was up all night on x-box’