The word 'key' in the term 'high key', is the 'key' to all this, it's a reference to the intensity of the key light, suggestive of it's use on the subject matter in the foreground.
photoshop has switched the meaning of 'dodge' and 'burn' but what we're talking about in 'over-' and 'under-' exposure is as regards the exposure of the film in the camera, so don't let this guy confuse you.
I assume Feltus is interjecting his take on exposure as the printing process is concerned. And it is what looks good that counts, right? Overexposure and underexposure are relative to your perception, as there is no 'correct' exposure for any shot, only an estimation of what exposure will show what you want to see in your final product. However, many times shots that are 'properly' exposed but not 'high-key' can be brightened and increased in contrast to create a 'hot' effect that looks good. Theoretically, this can be acheived with light backgrounds and colors and lots of light so the shot's not 'overexposed,' but just overall quite bright. Yes, high key is defined by some as a certain proportion of the tones in the overall shot being 'brighter' than a set luminosity. Meanwhile, let's talk about the question posed in this forum. A wise man once said, 'there is water at the bottom of the ocean.' Think on that for a while T+. However, based on the orientation of their heads to their feet, most of them would be upside down if you put moved them here without rotating them. People in Australia aren't upside down, in Australia. For all that, up and down are really relative too.
If you're shooting a positive print, slide, etc, the dark original is going to turn clear or white, so that when it's viewed you can't make any detail out of all or parts of the shot. Now, if you're shooting negative film, it's gonna basically turn the developed product black, so that the paper onto which you transfer your print is going to stay un(or "under-") exposed. Where I come from,overexposed implies: a) shooting with a shutter speed that is relatively long, allowing enough photons to penetrate the lens and fall on the film, that areas are created which lose visible detail on the film or positive print or b)an aperture that is (relatively) large is used such that the same happens.