pathologically incoherent, repetitious speech.

What does Logorrheic mean?

: excessive and often incoherent talkativeness or wordiness.

What do you call a person who never shuts up?

If someone's always mouthing off and just can't shut up, they've got logorrhea, a pathological inability to stop talking. ... As its sound suggests, logorrhea is related to diarrhea — an inability to stop something far more unpleasant from flowing.