You must legally get Entirely Normal Armchair into your hand before you can attempt to sneak it onto the battlefield. For example, you can't sneak it in from your sideboard or collection during a game.

Entirely Normal Armchair can be hidden in plain sight (i.e. just sitting there).

Your opponent may activate Entirely Normal Armchair's first activated ability even if they can't literally see it, provided they can identify where it is. That is, you can't just put your hand over it and claim that can't see. Well, you can claim that, and it might be true, but it won't do what you want it to.