Forming verbs and their derivatives.
[Latin de, off or from; less commonly via French dé-, Old French des-, from the Latin negative prefix dis-, in which it implies removal or reversal (see dis-).]
Examples of the first origin include decay (Latin de-, fall down or off, plus cadere, fall); defend (Latin de-, off, plus -fendere, to strike); and desist (Latin de-, down from, plus sistere, to stop). Examples of the second origin are defame (Latin dis-, expressing removal, plus fama, report); debate (Latin dis-, expressing reversal, plus battere, to fight); and deploy (Latin plicare, to fold).
The prefix has several meanings. In older words, adopted from French or directly from Latin roots, it can contain the idea of ‘down’ or ‘away’, often figuratively, as in descend, depress, degrade, and depose. Sometimes it implies something done completely or thoroughly, as with denude, devour, or derelict. It often has negative implications, as with deceive, delude, deride, and detest.
When de- is used to form verb on English stems, it has a sense of undoing the action of the stem verb—by removal, reversal, or separation—a sense closely related to un-: debrief, decertify, decriminalize, defrost, dehumidify, de-ice, delouse, desegregate, deselect. The prefix is often hyphenated before a vowel or in new creations. It is very active and examples from recent decades are de-archive, delayer (to reduce the number of levels in an organizational hierarchy of employees), deinstall (removal a piece of computer software from a system), demutualize (change the status of a mutual organization such as a building society to a different kind).