Hmmm. Well, I kinda did this. BUT I worked FT in the legal department of the university so the adjunct gig was like an extra credit thing (and also a favor to me because, come on - that's cool!). I did have to prepare, but not too much because it was my area of expertise and I'm great at presentations. I did not have to grade (online quiz, pass/fail 1 credit class). So, there you go - lol.
I don't even know how many times I've taught this class,but I still feel like I owe it to my students to look through my note before class. Maybe that doesn't count as true prep?
IDK. I certainly put thought into it before I taught and looked at my notes or updated slides. I cannot imagine ANYONE - even Trump - walking into a room cold (ETA: and also being successful). You might only need 10 or 20 minutes, but surely everyone does that?