loudyoudaoicibaDictYouDict[loud 词源字典]
loud: [OE] The underlying meaning of loud is ‘heard, audible’ – for it goes back ultimately to an Indo-European *klu- ‘hear’ (source also of English listen). The past participial form based on this, *klūtós, passed into prehistoric West Germanic as *khluthaz, which has since differentiated into German laut, Dutch luid, and English loud.
=> listen[loud etymology, loud origin, 英语词源]
loud (adj.)youdaoicibaDictYouDict
Old English hlud "noisy, making noise, sonorous," from West Germanic *khluthaz "heard" (cognates: Old Frisian and Old Saxon hlud, Middle Dutch luut, Dutch luid, Old High German hlut, German laut "loud"), from PIE past participle *klutos- (cognates: Sanskrit srutah, Greek klytos "heard of, celebrated," Armenian lu "known," Welsh clod "praise"), from root *kleu- "to hear" (see listen).

Application to colors first recorded 1849. The adverb is from Old English hlude, from Proto-Germanic *khludai (cognates: Dutch luid, German laut). Paired with clear since at least c. 1650.