A sexual act. Geoducking a girl is when you bang a girl in the ass. Just before you jizz, you pull out untill just the tip of your head is in. right after you jizz pull out and punch her right above her crack, making her ass tighten up and shoot the jizz out, just like a Geoduck.
"Dude, I geoducked that girl so hard last night, she hit the ceiling!"
"How pissed was your girlfriend after you geoducked her?"
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