case (n.1)youdaoicibaDictYouDict[case 词源字典]
early 13c., "what befalls one; state of affairs," from Old French cas "an event, happening, situation, quarrel, trial," from Latin casus "a chance, occasion, opportunity; accident, mishap," literally "a falling," from cas-, past participle stem of cadere "to fall, sink, settle down, decline, perish" (used widely: of the setting of heavenly bodies, the fall of Troy, suicides), from PIE root *kad- "to lay out, fall or make fall, yield, break up" (cognates: Sanskrit sad- "to fall down," Armenian chacnum "to fall, become low," perhaps also Middle Irish casar "hail, lightning"). The notion being "that which falls" as "that which happens" (compare befall).

Meaning "instance, example" is from c. 1300. Meaning "actual state of affairs" is from c. 1400. Given widespread extended and transferred senses in English in law (16c.), medicine (18c.), etc.; the grammatical sense (late 14c.) was in Latin. U.S. slang meaning "person" is from 1848. In case "in the event" is recorded from mid-14c. Case history is from 1879, originally medical; case study "study of a particular case" is from 1879, originally legal.[case etymology, case origin, 英语词源]
craven (adj.)youdaoicibaDictYouDict
early 13c., cravant, perhaps from Old French crevante "defeated," past participle of cravanter "to strike down, to fall down," from Latin crepare "to crack, creak." Sense affected by crave and moved from "defeated" to "cowardly" (c. 1400) perhaps via intermediary sense of "confess oneself defeated." Related: Cravenly; cravenness.
fall (v.)youdaoicibaDictYouDict
Old English feallan (class VII strong verb; past tense feoll, past participle feallen) "to drop from a height; fail, decay, die," from Proto-Germanic *fallan (cognates: Old Frisian falla, Old Saxon fallan, Dutch vallen, Old Norse falla, Old High German fallan, German fallen, absent in Gothic).

These are from PIE root *pol- "to fall" (cognates: Armenian p'ul "downfall," Lithuanian puola "to fall," Old Prussian aupallai "finds," literally "falls upon").

Meaning "come suddenly to the ground" is from late Old English. Of darkness, night, from c. 1600; of land sloping from 1570s; of prices from 1570s. Of empires, governments, etc., from c. 1200. Of the face or countenance from late 14c. Meaning "to be reduced" (as temperature) is from 1650s. Meaning "die in battle" is from 1570s. Meaning "to pass casually (into some condition)" is from early 13c.

To fall in "take place or position" is from 1751. To fall in love is attested from 1520s; to fall asleep is late 14c. To fall down is early 13c. (a-dun follon); to fall behind is from 1856. Fall through "fail, come to nothing" is from 1781. To fall for something is from 1903.

To fall out is by mid-13c. in a literal sense; military use is from 1832. Meaning "have a disagreement, begin to quarrel" is attested from 1560s (to fall out with "quarrel with" is from late 15c.).
occasion (n.)youdaoicibaDictYouDict
late 14c., "opportunity; grounds for action, state of affairs that makes something else possible; a happening, occurrence," from Old French ochaison, ocasion "cause, reason, excuse, pretext; opportunity" (13c.) or directly from Latin occasionem (nominative occasio) "opportunity, appropriate time," in Late Latin "cause," from occasum, occasus, past participle of occidere "fall down, go down," from ob "down, away" (see ob-) + cadere "to fall" (see case (n.1)). The notion is of a "falling together," or juncture, of circumstances.
Occident (n.)youdaoicibaDictYouDict
late 14c., "western part" (of the heavens or earth), from Old French occident (12c.) or directly from Latin occidentem (nominative occidens) "western sky, sunset, part of the sky in which the sun sets," noun use of adjective meaning "setting," from present participle of occidere "fall down, go down" (see occasion (n.)).
rickety (adj.)youdaoicibaDictYouDict
"liable to fall down," 1680s, from rickets (with + -y (2)), via notion of "weak, unhealthy." Literal sense is from c. 1720 but never common in English. Of material things, from 1799.
slide (v.)youdaoicibaDictYouDict
Old English slidan (intransitive, past tense slad, past participle sliden) "to glide, slip, fall, fall down;" figuratively "fail, lapse morally, err; be transitory or unstable," from Proto-Germanic *slidan "to slip, slide" (cognates: Old High German slito, German Schlitten "sleigh, sled"), from PIE root *sleidh- "to slide, slip" (cognates: Lithuanian slystu "to glide, slide," Old Church Slavonic sledu "track," Greek olisthos "slipperiness," olisthanein "to slip," Middle Irish sloet "slide").

Meaning "slip, lose one's footing" is from early 13c. Transitive sense from 1530s. Phrase let (something) slide "let it take its own course" is in Chaucer (late 14c.). Sliding scale in reference to payments, etc., is from 1842.
trundle (n.)youdaoicibaDictYouDict
"small wheel to support heavy weights," 1540s (implied in trundle bed "low bed on small wheels"), possibly from Middle English trendle "wheel, suspended hoop" (early 14c.), from Old English trendel "ring, disk" (see trend (v.)). Also probably in part from Old French trondeler "to roll down, fall down," which is of Germanic origin.
tumble (v.)youdaoicibaDictYouDict
c. 1300, "to perform as an acrobat," also "to fall down," perhaps from a frequentative form of Old English tumbian "dance about, tumble, leap." This is of unknown origin but apparently related to Middle Low German tummelen "to turn, dance," Dutch tuimelen "to tumble," Old High German tumon, German taumeln "to turn, reel." Transitive sense from late 14c. Related: Tumbled; tumbling.