beat (v.)youdaoicibaDictYouDict[beat 词源字典]
Old English beatan "inflict blows on, thrash" (class VII strong verb; past tense beot, past participle beaten), from Proto-Germanic *bautan (cognates: Old Norse bauta, Old High German bozan "to beat"), from PIE root *bhau- "to strike" (see batter (v.)). Of the heart, c. 1200, from notion of it striking against the breast. Meaning "to overcome in a contest" is from 1610s (the source of the sense of "legally avoid, escape" in beat the charges, etc., attested from c. 1920 in underworld slang).

Past tense beat is from c. 1500, probably not from Old English but a shortening of Middle English beted. Dead-beat (originally "tired-out") preserves the old past participle. Meaning "strike cover to rouse or drive game" (c. 1400) is source of beat around the bush (1570s), the metaphoric sense of which has shifted from "make preliminary motions" to "avoid, evade." Command beat it "go away" first recorded 1906 (though "action of feet upon the ground" was a sense of Old English betan). To beat off "masturbate" is recorded by 1960s. For beat generation see beatnik.[beat etymology, beat origin, 英语词源]
fend (v.)youdaoicibaDictYouDict
c. 1300, "defend, guard; protect; put up a fight; excuse or justify; forbid, bar," shortening of defend. From mid-14c. as "make a defense" and (usually with off (adv.)) "ward off, beat off, keep at a distance." Developed a meaning "make provision, give care" in Scottish English (16c.); hence to fend for oneself (1620s) "see to one's own defense." Related: Fended; fending.