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-.
1922, "to use an escalator," back-formation from escalator, replacing earlier verb escalade (1801), from the noun escalade. Escalate came into general use with a figurative sense of "raise" from 1959 (intrans.), originally in reference to scenarios for possible nuclear war. Related: Escalated; escalating. Transitive figurative sense is by 1962.
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<a href="https://www.etymonline.com/word/de-escalate">Etymology of de-escalate by etymonline</a>
Harper, D. (n.d.). Etymology of de-escalate. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/de-escalate