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-.
"oily fat of land animals," c. 1300, from Anglo-French grece, Old French gresse, craisse "grease, fat" (Modern French graisse), from Vulgar Latin *crassia "(melted) animal fat, grease," from Latin crassus "thick, solid, fat" (source also of Spanish grasa, Italian grassa), which is of unknown origin. Grease paint, used by actors, attested from 1880. Grease monkey "mechanic" is from 1918.
<a href="https://www.etymonline.com/word/degrease">Etymology of degrease by etymonline</a>
Harper, D. (n.d.). Etymology of degrease. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/degrease