"Stamp that dank," refers to the giving of the seal of approval to an act or a thing that has achieved greatness, aka: dankness.
This can also refer to the act of sleeping with or having sexual relations with an individual in a way similar to that indicated by the phrase: "Tap that."
This is not supposed to be derogatory, it is designed as a way for women to claim back the terms and phrases used against them by sleazeballs over the millennia.
That Quorn sandwich was really good, I 100% stamp that dank.
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"What did you do last night? I saw you go home with Person..."
"Yeah, they're sick. You know I stamped that dank."