1530s, "to lop off, take away or deduct a part of," from Medieval Latin defalcatus, past participle of defalcare, from de "off, away" (see de-) + Latin falx, falcem "sickle, scythe, pruning hook" (see falcate). Modern scientific use dates from 1808.
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-.
"hooked, curved like a scythe or sickle," 1801, from Latin falcatus "sickle-shaped, hooked, curved," from falcem (nominative falx) "sickle," which is of uncertain origin, perhaps a borrowing from a non-Latin Indo-European language of Italy. De Vaan lists cognates as Old Irish delg "thorn, pin," Welsh dala "sting," Lithuanian dilgė "nettle," Old Norse dalkr "pin, spine, dagger," Old English delg "clasp." Related: Falcated; falcation; falciform (1766).
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Harper, D. (n.d.). Etymology of defalcate. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/defalcate