When a drug dealer either sells someone less of a drug that they paid for or passes off another substance as the drug someone is buying.
I bought an eighth of weed but the bag was missing almost a gram, the bastard sold me a wack sack.
I bought a gram of coke but it was just baking soda, i was tricked with a wack sack.
a large amount. A collective noun of more than the usual or normal quantity
The Duggars have a wack sack of kids.