Logorrhea

Logorrhea:                  excessive and often incoherent talkativeness or wordiness.

First encountered:     watching a crappy* film 'Life or something like it' out of the corner of my ear. Liked the word, looked it up.

How it's used:            In common parlance it can be used anwhere 'verbal diarrhea' might be used. For example:' Logorrhea is a useful personality trait for anyone wanting to go into politics.'
In Pscychology Logorrhea is a communication disorder sometimes classified as a mental illness, resulting in incoherent talkativeness.(thanks wikipedia)
However as one of the symptoms can be monotous, incoherent mumbling, it might be hard to tell the diference between a legitimate sufferer and someone under the influence of drugs, alcohol or whatnot.
How you can use it:    'You never stop going on. Have you got logorrhea or summit';

Origin:                       its all greek to me. Sounds like slang - verbal diarrhea and all that, also possibly could come from logos meaning 'word' and rhoia meaning 'flow'

* IMHO but Not just my opinion - http://www.rottentomatoes.com/m/life_or_something_like_it/