late 14c., "peaceable, being in a state of rest, restful, tranquil, not moving or agitated," from Old French quiet and directly from Latin quietus "calm, at rest, free from exertion," from quies (genitive quietis) "rest" (from PIE root *kweie- "to rest, be quiet").
From 1510s as "peaceable, not turbulent, characterized by absence of commotion." By 1590s as "making no noise." From 1570s as "private, secret." As an adverb from 1570s. Quiet American, frequently meaning a U.S. undercover agent or spy, is from the title of Graham Greene's 1955 novel. Related: Quietly; quietness.
word-forming element making verbs (such as darken, weaken) from adjectives or nouns, from Old English -nian, from Proto-Germanic *-inojan (also source of Old Norse -na), from PIE adjectival suffix *-no-. Most active in Middle English and early modern English, hence most verbs in -en are comparatively recent.