So it does kind of fit in with the other definitions: download is from the wider internet down to your local device, upload is from your local device up to the wider internet, so sideload is just moving something from somewhere local to somewhere else local. I imagine sideload wasn’t generally used before because we’d just say “copy/install a file” or similar, and its usage now comes from it being a shorthand for the slightly convoluted process required on mobile devices.