"relieve or reduce pressure," by 1866, from de- + compress (v.). In early use especially "restore gradually to normal conditions of air pressure." Figurative sense "become calm, relax" is by 1964. Related: Decompressed; decompressing.
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-.
late 14c., "to press or pack (something) together, force or drive into a smaller compass," from Old French compresser "compress, put under pressure," from Late Latin compressus, past participle of compressare "to press together," frequentative of comprimere "to squeeze," from com "with, together" (see com-) + premere "to press, hold fast, cover, crowd, compress" (from PIE root *per- (4) "to strike"). Related: Compressed; compressing. Compressed air is attested from 1660s.
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Harper, D. (n.d.). Etymology of decompress. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/decompress