to get really high; super baked off your ass; to have smoked so much marijuana that you in fact believe you're slapping a leprechaun everytime you take a hit.

slapping the leprechaun in the face would mean you have the first hit.
slapping the leprechaun in the stomach or chest would be taking hits in between the first and last
slapping the leprechaun in the butt would mean taking the last hit of that bowl, joint, or device you're using to smoke out of.

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