I absolutely agree that it's not OK for a professional to ask a question like that. An intern, maybe. I also think, however, that there are better ways to deal

I absolutely agree that it's not OK for a professional to ask a question like that. An intern, maybe. I also think, however, that there are better ways to deal with it. For example, if someone has never heard of Lisp, go ahead and explain it, but make a mental note of it. If you continue to find them asking questions that suggest they spent most of their development career under a rock, present those concerns to your employer. Help them make a decision whether or not this person is able to perform at the level they need to. If it's a one-off thing though, and they seem otherwise fairly competent, then it's a good thing you didn't act like a jerk because now the person doesn't resent you, and they won't have to ask you what Lisp is again. Source: I've worked with several developers at a level much lower than I expected, this was in one case how I wished I had reacted, and in two other cases how I did and it worked out well.