Longman Dictionary of Contemporary English
The Collaborative International Dictionary
De- \De-\ A prefix from Latin de down, from, away; as in debark, decline, decease, deduct, decamp. In words from the French it is equivalent to Latin dis- apart, away; or sometimes to de. Cf. Dis-. It is negative and opposite in derange, deform, destroy, etc. It is intensive in deprave, despoil, declare, desolate, etc.
Douglas Harper's Etymology Dictionary
active word-forming element in English and in many words 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), etc. Compare also dis-.
Wiktionary
pre. 1 Meaning reversal, undoing or removing. 2 intensify. 3 Meaning from, off.