"not present or found, absent," 1520s, present-participle adjective from miss (v.). Military sense of "not present after a battle but not known to have been killed or captured" is from 1845. As a noun by 1855.
Missing link was used in various figurative senses before is attested 1846 in reference to forms of plant and animal life [Chambers, "Vestiges of the Natural History of Creation"]; in reference to a hypothetical creature between man and the apes, as a component of Darwin's theory of evolution, by 1860.
But then, where are the missing links in the chain of intellectual and moral being? What has become of the aspirants to the dignity of manhood whose development was unhappily arrested at intermediate points between the man and the monkey? It will not be doubted, we presume, that there exists at present an enormous gap between the intellectual capabilities of the lowest race of men and those of the highest race of apes; and if so, we ask again, why should the creatures intermediate to them—exalted apes or degraded men—have been totally exterminated, while their less worthy ancestors have successfully struggled through the battle of life? [William Hopkins, "Physical Theories of the Phenomena of Life," Fraser's Magazine, July, 1860]
In a popular religious tract of the late 1850s, "The Missing Link of the London Poor," the missing link was the Bible. Missing person, one who has disappeared and whose condition, whether alive or dead, is unknown, is by 1820.
c. 1600, "group of students," in U.S. especially "number of pupils in a school or college of the same grade," from French classe (14c.), from Latin classis "a class, a division; army, fleet," especially "any one of the six orders into which Servius Tullius divided the Roman people for the purpose of taxation;" traditionally originally "the people of Rome under arms" (a sense attested in English from 1650s), and thus akin to calare "to call (to arms)," from PIE root *kele- (2) "to shout." In early use in English also in Latin form classis.
Meaning "an order or rank of persons, a number of persons having certain characteristics in common" is from 1660s. School and university sense of "course, lecture" (1650s) is from the notion of a form or lecture reserved to scholars who had attained a certain level. Natural history sense "group of related plants or animals" is from 1753. Meaning "high quality" is from 1874. Meaning "a division of society according to status" (with upper, lower, etc.) is from 1763. Class-consciousness (1903) is from German Klassenbewusst.
The fault, the evil, in a class society is when privilege exists without responsibility and duty. The evil of the classless society is that it tends to equalize the responsibility, to atomize it into responsibility of the whole population—and therefore everyone becomes equally irresponsible. [T.S. Eliot, BBC interview with Leslie Paul, 1958]
1705, "to divide into classes, place in ranks or divisions," from class (n.) or French classer. Sense of "to place into a class" is from 1776. Related: Classed; classing.
"belonging to the class next after the first," 1833, from the noun phrase (1810) indicating the second of a ranked series of classes (originally in a university, later of railroad accommodations, etc.), from second (adj.) + class (n.). The phrase second-class citizen is recorded from 1942 in U.S. history.
The Negro recognizes that he is a second-class citizen and that status is fraught with violent potentialities, particularly today when he is living up to the full responsibilities of citizenship on the field of battle. [Louis E. Martin, "To Be or Not to Be a Liberal," in The Crisis, September 1942]
1766, in a British sense, "class of people socially intermediate between the aristocratic and the laboring classes, the community of untitled but well-bred or wealthy people," from middle (adj.) + class (n.). As an adjective, "pertaining to the middle class," by 1857, with reference to education. Nares reports menalty as an early word for "the middle class" (1540s).