active word-forming element in English and in many verbs inherited from French and Latin, from Latin de "down, down from, from, off; concerning" (see de), also used as a prefix in Latin, usually meaning "down, off, away, from among, down from," but also "down to the bottom, totally" hence "completely" (intensive or completive), which is its sense in many English words.

As a Latin prefix it also had the function of undoing or reversing a verb's action, and hence it came to be used as a pure privative — "not, do the opposite of, undo" — which is its primary function as a living prefix in English, as in defrost (1895), defuse (1943), de-escalate (1964), etc. In some cases, a reduced form of dis-.

the usual form of Latin dis- in Old Italian, Spanish, Portuguese, Provençal, and French. In Middle English, interchanging with dis- (which in a later period displaced it), hence such form as desarmen, desdein, deshonour, desparagen, deschargen, despisen, etc.

in the metric system, word-forming element denoting one-tenth of the standard unit of measure, 1801, from French deci-, taken arbitrarily from Latin decimus "tenth," from decem "ten" (from PIE root *dekm- "ten").

word-forming element meaning "half, half-sized, partial," used in English from mid-14c., especially in technical terms from French, from Old French demi "half" (12c.), from Late Latin dimedius, from Latin dimidius "half, one-half," which contains the elements dis- "apart" (see dis-) + medius "in the middle, between; from the middle," as a noun "the middle;" from PIE root *medhyo- "middle." Formerly also demy-, and in early use often written as a separate word.

also desoxy-, word-forming element used to make chemical names for compounds which contain fewer oxygen atoms than other compounds, from de- + first two syllables of oxygen.

word-forming element meaning "of or pertaining to skin," from Greek dermat-, from derma "(flayed) skin, leather," from PIE root *der- "to split, flay, peel," with derivatives referring to skin and leather. The shortened form derm- was used from mid-19c. but is considered incorrect.

before vowels desm-, word-forming element used in scientific compounds and meaning "band, bond, ligament," from Greek desmos "bond, fastening, chain," related to dein "to bind," from PIE root *dē- "to bind."  

before vowels deuter-, word-forming element meaning "second," from Late Latin deutero-, from Greek deuteros "next, second," a word of uncertain origin. According to some sources from duo "two" (from PIE root *dwo- "two"), but according to Watkins the ground sense is "missing" and the Greek word is from PIE *deu-tero-, suffixed form of *deu- (1) "to lack, be wanting." But Beekes doubts even this. 

word-forming element of Latin origin meaning 1. "lack of, not" (as in dishonest); 2. "opposite of, do the opposite of" (as in disallow); 3. "apart, away" (as in discard), from Old French des- or directly from Latin dis- "apart, asunder, in a different direction, between," figuratively "not, un-," also "exceedingly, utterly." Assimilated as dif- before -f- and to di- before most voiced consonants.

The Latin prefix is from PIE *dis- "apart, asunder" (source also of Old English te-, Old Saxon ti-, Old High German ze-, German zer-). The PIE root is a secondary form of *dwis- and thus is related to Latin bis "twice" (originally *dvis) and to duo, on notion of "two ways, in twain" (hence "apart, asunder").

In classical Latin, dis- paralleled de- and had much the same meaning, but in Late Latin dis- came to be the favored form and this passed into Old French as des-, the form used for compound words formed in Old French, where it increasingly had a privative sense ("not"). In English, many of these words eventually were altered back to dis-, while in French many have been altered back to de-. The usual confusion prevails.

As a living prefix in English, it reverses or negatives what it is affixed to. Sometimes, as in Italian, it is reduced to s- (as in spend, splay, sport, sdain for disdain, and the surnames Spencer and Spence).

word-forming element of Greek origin meaning "two, double, twice, twofold," from Greek di-, shortened form of dis "twice," which is related to duo "two" and cognate with bi-, from PIE root *dwo- "two." In chemistry it indicates a compound containing two units of the element or radical to which it is prefixed.