Which way is upside?

Which way is upside?

In the US, when somebody slaps you "upside your head", is that a downward blow from above, or an upward blow from below? The word isn't used in the UK.

I don't think it really matters. IMO, upside in this phrase is not directional but positional - it basically means the same as beside, alongside, against the side of. I would expect a straight across slap against the side of the head.

If both the slapper and the slappee are standing and they're about the same height, the direction of bringing the hand up to head level would be upward.

☺ As a Yankee, I hope our British friends do not get the notion that "upside the head" is a more common expression than it actually is. Doubtless it is or was a common enough expression somewhere in that vast area east of the Mississippi River and south of the Mason-Dixon Line (that is, south of Pennsylvania). It has found a use elsewhere in American speech and writing only because it is regarded as picturesque, or "cute." And if one or more of my countrymen think me wrong, speak up.
W Z

I think it originated in Black English, which correlates with the South. I expect most males I associate with to have heard it.

For a little context, and for evidence that it can mean more than one simple slap, take a look at these lines from Snoop Dogg's "Snoop's Upside Yo' Head":

"I think it's bout' time that we hit these [word removed in order to comply with Google's Publisher Policy]s upside
the head with some of that west coast
gangsta' sh......... Ooh.....Wee!!!!!!!!!!"