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1.
To extend your arm so that your hand is below the back of a person's head and thrust your hand upwards so that your palm hits them square in the back of the head.

This is typically done when someone does something stupid or just to humiliate/annoy them.
When Rose started to take diet pills, her mother gave her a smack upside the head to knock some sense into her.

As John sat in the passenger seat, the driver acted like her was streching, but really smacked John upside the head and hillarity ensued.
by ByRd May 08, 2007