"What are you playing off now?"
The correct answer to this question is "what the f*&$ is it to you, %^&*-face?".
Unless the person asking is marking your card and you have neglected to fill in your handicap, there's no excuse for this kind of rudeness. A certain person I play with occasionally, however, never fails to ask, whether or not she's marking my card. Even if we're not playing together. Even if we're not playing in the same competition, but just happen to be standing next to each other at the bar. I don't know why she thinks it's acceptable to just walk up to someone and ask them what their handicap is. I don't ask her her dress size, or "how much do you weigh these days?".
A golfer with manners doesn't need to ask because they don't need to know. How is my handicap relevant to you, unless you're marking my card? It's not. The whole point of handicapping is that my nett or stableford result is entirely equivalent to yours, even if our handicaps are 36 shots away from each other's. "I had a nett 73" gives you enough information, in the same way as "I lost 2lbs this week" tells you what you need to know without divulging the results of my latest Weightwatchers weigh-in.
The only reason someone would ask your handicap is to judge you. "He plays off 8 so he must be good. She plays off 29, she must be shite." In the meantime, we've all played with a single-figure handicapper who's a complete wanker, thinks they're a few practice rounds away from the European Tour, but despite a shaky grasp of the Rules is willing to argue the toss about whether their opponent is allowed to blow their nose during matchplay. Conversely, the 29 handicapper may be ex-Captain, County match referee, and the most encouraging companion to have around 18 holes. Judging someone by their handicap is... unreliable.
That's not to say that it's not tempting to ask sometimes. When someone you know at work turns out to be a golfer, it would be strange not to be curious about what level they play at. Nevertheless I generally try to refrain from asking their handicap, since what I really want to know is how long someone has played for, and how regularly.
Some people are just rude though. When this certain person asked me this week, my "why do you want to know?" wasn't enough, and she persisted until I actually gave her the number. Frankly she could just have looked on the board in the locker room. Next time she asks I'll have to be ready with a "what's your bra size?".