"a great number regarded collectively; a crowd or throng; the characteristic of being many, numerousness," early 14c., from Old French multitude (12c.) and directly from Latin multitudinem (nominative multitudo) "a great number, a crowd; the crowd, the common people," from multus "many, much" (see multi-) + suffix -tudo (see -tude). Related: Multitudes.

A multitude, however great, may be in a space so large as to give each one ample room; a throng or a crowd is generally smaller than a multitude, but is gathered into a close body, a throng being a company that presses together or forward, and a crowd carrying the closeness to uncomfortable physical contact. [Century Dictionary]

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.

"an indefinitely large number; a crowd, many persons," Old English menigu, from a prehistoric Germanic word from the source of many (adj.). Compare Old Saxon menigi, Gothic managei "multitude, crowd," Old High German managi "large number, plurality," German Menge "multitude." The many "the multitude, the mass of people, the common herd" is attested from 1520s.  

1570s, "the common people," from French vulgarité and directly from Late Latin vulgaritas "the multitude," from vulgaris (see vulgar). Meaning "coarseness, crudeness" is recorded from 1774.

"state of being numerous," 1610s, from Latin numerositatem (nominative numerositas) "a great number, a multitude," from numerosus "numerous," from numerus "a number" (see number (n.)).

1610s, "whole number of inhabitants in a country, state, county, town, etc," from Late Latin populationem (nominative populatio) "a people; a multitude," as if from Latin populus "a people" (see people (n.)). From 1776 as "act or process of peopling" (a country, etc.). Population explosion "rapid or sudden increase in the size of a population" is attested by 1953.

"of or characteristic of the lower class or the common people," 1560s in a Roman historical sense, from Latin plebeius "belonging to the plebs," earlier plebes, "the populace, the common people" (as opposed to patricians, etc.), also "commonality; the mass, the multitude; the lower class" (from PIE *ple-, from root *pele- (1) "to fill"). In general (non-historical) use from 1580s.