an insult used frequently or as a light hearted diss to distract, "put-off", or annoy freinds and co workers. The word refers to a gentlemans scrotum.
from distance a freind is tuning a girl, you yell "sack!" as loudly as possible or "peter is a sack!". It is best used in a drive-by sense. Or when someone is about to snap to get maximum effect.