You use it as an insult to someone. Especially meaningful when is combined with "jtlyk".
"Jtlyk, he is not a spag."
"Stop being so spaggy!"
"Deena and Anna aren't spags."
By Lillis
Spag
spag = gwyns word that means anything you want it to mean. ( you spag) can be used as a light joke or an insult. it origonates from the australian cultures calling the italianspags though to be truthfull.