c. 1600, "of vast extent;" 1620s, "consisting of a great number," from Latin multitudin-, stem of multitudo (see multitude) + -ous. First in Shakespeare or Dekker, depending on the dating of their publications, though it is certainly "Macbeth" that has fixed it in the language. Related: Multitudinously; multitudinousness.
Will all great Neptune's ocean wash this blood
Clean from my hand? No; this my hand will rather
The multitudinous seas incarnardine,
Making the green one, red.