Lewis & Short

mussĭtātor, ōris, m. [mussito],

  1. I. a silent person, a mutterer, Vulg. Isa. 29, 24; cf. mussitator, ὀπογογγυστής, Vet. Gloss.
  2. II. A grumbler: mussitatores discent legem, Vulg. Isa. 29, 24.