I always tell my students to bring a pocket dictionary (some of them have it on their mobile phones). But most of time sts feel too lazy to use it,  or they may not understand the meaning of a word even if they look it up; so in both cases they prefer to ask the teacher which seems easier for them.

So if I don't know the meaning of a word I just tell them " Young ladies and gentlemen, your friend is asking about the meaning of this word so take out your dictionaries and let's see who is going to be the first to find it!". This gives me time to look up the word in my own dictionary or I can only listen to their definitions which helps  me give further explanation if needed.

Interesting ideas.

I've signed up to dictionary.com's word of the day. They arrive by email each day. They're usually words that would never come up in class but there are some can be useful.

Otherwise, honesty is often the best policy, however embarrassing it is. Admit you don't know and say you'll let them know. We're not lexicographers, are we?