mod n.1

c. 1500, "now existing;" 1580s, "of or pertaining to present or recent times;" from French moderne (15c.) and directly from Late Latin modernus "modern" (Priscian, Cassiodorus), from Latin modo "just now, in a (certain) manner," from modo (adv.) "to the measure," ablative of modus "manner, measure" (from PIE root *med- "take appropriate measures"). Extended form modern-day attested from 1872.

In history, in the broadest sense, opposed to ancient and medieval, but often in more limited use. In Shakespeare, often with a sense of "every-day, ordinary, commonplace." Meaning "not antiquated or obsolete, in harmony with present ways" is by 1808.

Of languages, indicating the current form of Greek, etc., 1690s; modern languages as a department of study (1821) comprised those now living (i.e. not Latin or Greek) that were held to have literary or historical importance. The use of modern English is at least from c. 1600 (in Cowell's "Interpreter," explaining an Anglo-Saxon word). The scientific linguistic division of historical languages into old, middle, and modern is from 19c.

Slang abbreviation mod is attested from 1960. Modern art is from 1807 (in contrast to ancient; in contrast to traditional, representing departure or repudiation of accepted styles, by 1895); modern dance is attested by 1912; modern jazz by 1954. Modern conveniences is recorded by 1926.

c. 1500, in philosophy, "determination by a mode or quality," from French modification (14c.) and directly from Latin modificationem (nominative modificatio) "a measuring," noun of action from past-participle stem of modificare (see modify). Sense of "a result of a variation or alteration" is from 1660s. Meaning "act or process of altering in character, form, or function" is from 1774.

1852, "a rocking chair," American English, agent noun from rock (v.1). Middle English had rokker, "nurse charged with rocking a cradle" (early 14c.). In sense of "one of the curved pieces of wood that makes a chair or cradle rock" it dates from 1787. Meaning "one who enjoys rock music" (opposed to mod (n.1)) is recorded from 1963, from rock (v.2).

Slang off (one's) rocker "crazy" is attested by 1897 according to OED; a widely reprinted 1903 newspaper column in U.S. identified it as British slang; the image is perhaps mechanical. To get (off) one's rocker seems to have been used earlier in U.S. baseball slang for "get busy, get active in a game" (1895) and does suggest the rocking-chair.