middle class n.

Old English middel, "equally distant from extremes or limits; intermediate," from Proto-West Germanic *midla- (source also of Old Frisian middel, Old Saxon middil, Middle Low German, Dutch middel, Old High German mittil, German mittel), from Proto-Germanic *medj, from PIE root *medhyo- "middle."

Middle finger "the third finger" (counting the thumb as the first)  so called from late Old English. Middle school is attested from 1838, originally "middle-class school, school for middle-class children;" the sense in reference to a school for grades between elementary and high school is from 1960. Middle management, the level below senior management, is by 1941.

Middle-of-the-road in the figurative sense is attested from 1894, originally political; edges of a dirt road can be washed out and thus are less safe, but the notion here probably is of the middle as "less exposed to ambush." Middle way in the figurative sense of "path of moderation" is from c. 1200. Middle ground as "place of moderation or compromise between extremes" is by 1961. Middle-sized "of medium size" is by 1620s.

In U.S. history, the Middle States (1784) were those between New England and the South (New York, New Jersey, Pennsylvania, Delaware). Middle America for "the 'silent majority,' the generally conservative middle regarded as a homogeneous group" is by 1968.

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 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]