Taking their stack refers to when
one individual or group takes the possessions of someone else/ group. It can be used in any context not just materialistic such as when you effortlessly outsmart someone in a game or competition, leaving them with
nothing but a metaphorical stack of defeat, while you bask in the glory of victory and maybe even a little smugness.
An example: Two individuals (are negotiating a contract with two sporting clubs,
one offering $
50,000 net and another offering $55,000 gross. They
don't
check properly and go with the $55,000 contract. These two individuals tell their mates about the first club offer and decide to take the $50,000 contract. The original two individuals
soon realise gross mean before tax leaving them with a mear $35,000. In this case the second two people "took their stack"