Mouthful Of Marbles Meaning

(idiomatic) An indistinct, muffled or garbled manner of speaking.

Example: 2001, Timothy Erickson et. al., "Toxicology Update: A Rational Approach to Managing the Poisoned Patient," Emergency Medicine Practice, vol. 3, no. 8, p. 5,
  Anticholinergic toxicity results in a characteristic mumbling, as if the patient is trying to quickly recite a haiku with a mouthful of marbles.