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)