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

In classical Latin, dis- paralelled 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 new 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.