Stacks at the Apple store, man this ballin' is new to us
Stacks is a common term used to describe large amounts of money. Childish is saying that ballin' on a high enough level that walking into an Apple store with stacks of cash is a new thing to them (his crew). The Apple store, filled with high-end gadgetry, is generally fairly expensive.
But not for Childish. Because he’s got stacks.
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