Sideloading is a term that’s been around for decades, it’s not some made up word by tech giants to make people scared of installing apps.
The term originates from a designation for transferring data between physical devices and was slowly adopted (because language is fluid) to its current definition (by people on forums like xda).
This isn’t some conspiracy and Google and apple don’t need to use coded language to prevent you from side loading, apple for example just outwardly and bluntly forbids it.
I think any unknown phrase and method to install an app will be scary to a person who is that unknowledgeable about it. At that point there isn’t any phrase that you could use that wouldn’t sound sketchy to them, it isn’t the phrase that is the problem, it’s the fact that it’s unknown and the process is scary.
The people you are describing would still be skeptical even if you explained it to them (and they should be, since they likely don’t have the knowledge or resources to properly vet an application from an unknown source)