You also have to watch for the attempt to extract a reason from you. Sometimes it'll be a bald "why not?" but sometimes it's more subtle like "Oh, spending more time with the family, huh?" trying to leave you socially obligated to affirm, deny, or correct it. I find a very neutral "mmmm" to be a pretty good disengagement; you clearly heard them, but it doesn't affirm, deny, correct, or really leave any opening for them to wedge into without crossing over the politeness line to the point that you can openly call them out if they keep pursuing it.

I'm increasingly prone to "none of your f------ business", though usually only at the 2nd or subsequent query repeat.

Prying is socially hostile and should be responded to as such.

In certain cases of ongoing, and contentious, relationships, sooner.

I took a communication class many years ago and this was the main thing I took away from it. If you give a reason or an excuse the other person will try to help you "solve" it. If you give a simple "no, sorry" or "no, thanks" there's nothing for them to latch onto or argue with.

Realize that the other person is free to ask anything they want, but just the act of them asking doesn't create any obligation on your part. Especially with people where you don't have a longstanding relationship, you don't owe anyone anything more than a polite no.

The other piece I learned was this useful phrase to deploy when under severe pressure to give a reason and for whatever reason you can't just disengage or leave: "Sorry, I'm just not comfortable with that". No one can reasonably argue with that, and if they do, having already given your reason you can then simply fall back into a loop of "No, sorry."

"I'm not comfortable with that" invites them to ask why you aren't comfortable, what they could do to make you more comfortable, etc. If you genuinely can't leave, you'll just have to change the subject or hold your ground. Don't underestimate the power of silence, which is to say blankly staring at them while they attempt to press you (if they're being plainly obnoxious).

You took communication class and main thing you got out of it was how to not to communicate?

It's like going to knife-fighting class to learn how to win every knife fight and receive "buy a gun" advice. Yes, it works. But it's not really a knife-fighting skill, is it?

That's a great example you've used here, because number one lesson of winning a knife fight is, run. The best alternative to running away or otherwise avoiding that fight would be indeed to shoot your way out of it.

WRT GP's comment, what they described is communication. It's precise communication. Saying exactly what you mean and nothing more. Not falling to some kind of instinctive save-face pressure and inventing a bullshit reason, that only leaves you worse off (you now either have to defend from your interlocutor's attempts at helping you, or admit that you've lied).

(To use a knife-fighting analogy, it's like learning to thrust without making it easy for your opponent to dodge your attack and cut you up.)

This is a hilarious example because knife fighting “skill” is Hollywood BS. Any sort of close quarters combat instruction will nearly always focus on getting out of any altercation involving knives.

There’s a saying about bringing a gun to a knife fight, or a knife to a gunfight, I can’t remember which but the guy with the gun wins. Usually.