word-forming element meaning "down, downward," but also "through, on, against, concerning," etc., from Latinized form of Greek kata-, before vowels kat-, from kata "down, downward, down from, down to," from PIE *kmt- "down, with, along" (source also of Hittite kattan (adv.) "below, underneath," katta "along with"). Occasionally in Greek it had senses of "against" (catapult) or "wrongly" (catachresis), also "along, through, over, across, concerning." Also sometimes used as an intensive or with a sense of completion of action (catalogue). Very active in ancient Greek, this prefix is found in English mostly in words borrowed through Latin after c. 1500.

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-.

word-forming element meaning "toward; against; before; near; across; down," also used as an intensive, from Latin ob (prep.) "in the direction of, in front of, before; toward, to, at, upon, about; in the way of; with regard to, because of," from PIE root *epi, also *opi "near, against" (see epi-).

word-forming element meaning "killer," from French -cide, from Latin -cida "cutter, killer, slayer," from -cidere, combining form of caedere "to strike down, chop, beat, hew, fell, slay," from Proto-Italic *kaid-o-, from PIE root *kae-id- "to strike." For Latin vowel change, see acquisition. The element also can represent "killing," from French -cide, from Latin -cidium "a cutting, a killing."

before vowels seism-, word-forming element meaning "earthquake," from Greek seismos "a shaking, shock; an earthquake," also "an extortion" (compare colloquial shake (someone) down), from seiein "to shake, agitate, sway; to quake, shiver" from PIE root *twei- "to agitate, shake, toss; excite; sparkle" (source also of Sanskrit tvesati "to excite; to be excited, inflame, sparkle" and words in Avestan for "fears" and "fright, danger").

before vowels chaet-, word-forming element meaning "hair," also, in scientific use, "spine, bristle," from Latinized form of Greek khaitē "long, loose, flowing hair" (of persons, also of horses, lions), from an old PIE word for "hair, mane," source also of Avestan gaesa- "curly hair," gaesu- "'curly haired," Modern Persian ges "hair that hangs down, curls;" Middle Irish gaiset "bristly hair."

word-forming element meaning "away, from, from off, down," denoting disjunction, separation, departure; from Latin ab (prep.) "off, away from" in reference to space or distance, also of time, from PIE root *apo- "off, away" (also the source of Greek apo "off, away from, from," Sanskrit apa "away from," Gothic af, English of, off; see apo-).

The Latin word also denoted "agency by; source, origin; relation to, in consequence of." Since classical times usually reduced to a- before -m-, -p-, or -v-; typically abs- before -c-, -q-, or -t-.

a sound represented in Old English by -sc- (fisc "fish"), which originally was pronounced "-sk-" but which by late Old English had softened to "-sh-." Modern English words with -sc- mostly are imports (generally Scandinavian).

The "sh" sound did not exist in Old French, therefore French scribes in England after the Norman conquest often represented it with -ssh- in medial and final positions, and sch- in initial positions (schape, schamful, schaft for shape, shameful, shaft). But the spelling -sh- has been standard since Caxton, probably as a worn-down form of Middle English -sch-.

In some East Anglian texts from 14c.-15c., x- is used (xal, xulde for shall, should), which would have given the language a very different look had it prevailed, but the London-based sh- ended up as the standard form. The same Germanic sound has become, by natural evolution, modern German and Dutch sch-, Scandinavian sk-.