Diarrhea of the mouth. The sudden out-pouring of words. Basically, you're long-winded, and have trouble cutting down on words whenever you write, talk, or communicate in any other way.
Teacher: "....when writing a timed write you want to get to your point as quickly as possible without losing your reader."
Student: "But what if we have a bad case of logarhea?"
Teacher: "That's when you better hope you have plenty of time left because otherwise you're screwed."