Shoutbox

A double click can be detected by subclassing the chat window. Then you can use Plus! Script's functions do detect the selected word (when you double click a word it is selected), hence you can retrieve the word.

There are plenty of dictionnaries on the net you can use, there are even ActiveX dictionnaries and such which can do the looking up for you and which simply give back a list of words or whatever...

In other words, this is very doable. The question is if anybody is willing to spend some time to make it (and doing it in a proper decent way) as it apparently isn't considered very usefull by many people and the novelty would wear off quickly I guess.