I always thought of it as coming from side-channel. Which (until I searched just now and was only offered side channel attacks as a result) I generally construed it as a good thing because the system was assumed to be broken. Things like track 2 diplomacy or messaging the CEO/minister because customer service/bureaucracy was broken. You can go in the side door of a business if you own it or belong there, only dis-empowered customers are forced to go in the front.

Side loading was getting something to work because it should when the system hadn't caught up to the fact that it should work.