Don't regard "Did you look that up in a dictionary?" as a rude question. Take it at face value. It's a hint, a prompt.
Either the O.P. used a dictionary first, or not.
If not, a dictionary should have been consulted first. (Probably a few dictionaries, actually.)
If so, then results from that prior research should have been included, summarized, or at least mentioned in the question. Potentially, that can serve a few functions:
- It demonstrates prior research was done, which in turn helps ward off negative comments and downvotes.
- It helps people reading the question better understand where the O.P. is coming from, and where the confusion lies.
- It saves people who want to answer the question a trip to the dictionary themselves.
- It prevents the answers from going in several different directions that the O.P. doesn't want by clarifying what the question is really asking about.
In short, it's not a rude question, it's a fair question.
There's two sides to the rudeness coin. Personally, I think there's a degree of "rudeness" when a question is skimpy on details and scant on research. It's like saying, “I just want an answer to my question. I don't want to have to do any work myself, or put any effort into this. Everyone should just be able to figure out by osmosis what I already know, and what I still need to figure out.”
By the way, there's a reason I put "rudeness" in scare quotes. I don't think most new users are really being "rude" – certainly not intentionally. We live in an online society filled with tweets and snarky comments, where brevity is valued and thoughtful deliberateness is shrugged off. Newcomers to the Stack Exchange often don't realize that this site strives to be differernt.
Every help page on SE starts with:
We're a little bit different from other sites.
and exorts new users to:
Include details about what you have tried and exactly what you are trying to do.
In a language forum, that "details about what you have tried" part means, “Did you look this up? What did you search for? What did you find?” – particularly when the question is primarily about the meaning of words or phrases.
As for how to pull this off and write good questions, I've already written about this in an answer to an ELL meta question. And ELL is probably where most "ESL students, general non-English speaking persons seeking clarification" should ask their questions – but that's another matter entirely.