Megagaltastic means "possessing a large vocabulary."
In a sentence: "Greg always knows the definition of words I don't know; he's quite megagaltastic."
The term was coined by the web personality Ze Frank in a video blog on October 19th, 2006, who noted that while there are multiple words for "using a large vocabulary" (loquacious, eloquent, voluble), there's no word in English for "having a large vocabulary". (This is an example of a lexical gap.) Ze petitioned the Merriam Webster dictionary to add it, and while they agreed that they knew of no word with that precise definition, the word would have to make its way into more sources before it could be accepted. This is one such source.