What does 'burden bore' from 'The Raven' mean?

The whole stanza is referring to the only Word the raven learned from its former "unhappy master", and that Word "is its only stock and store". The former "master" spoke "dirges" (laments or mournful songs) of "hope" (read sarcasm here) often and ever increasingly of every "unmerciful Disaster" until the Word nevermore became "one/melancholy burden bore" by the raven.