letyoudaoicibaDictYouDict[let 词源字典]
let: [OE] English has two distinct verbs let, of diametrically opposite meaning, but they are probably ultimately related. The one meaning ‘allow’ goes back to a prehistoric Germanic *lǣt- (source also of German lassen and Dutch laten) which, like the related late, is connected with a range of words denoting ‘slowness’ or ‘weariness’. It therefore appears that the underlying etymological meaning of let is ‘let go of something because one is too tired to hold on to it’.

By the time the verb reached Old English this had developed to ‘leave behind’ and ‘omit to do’, senses now defunct, as well as to ‘allow’. A close relative of the base *lǣt- was *lat-, direct ancestor of English late. From this was formed the Germanic verb *latjan, which gave English its other verb let, meaning ‘prevent’, now largely obsolete except as a noun, in the phrase without let or hindrance or as a tennis term.
=> late[let etymology, let origin, 英语词源]
let (v.)youdaoicibaDictYouDict
Old English lætan "to allow to remain; let go, leave, depart from; leave undone; to allow; bequeath," also "to rent" (class VII strong verb; past tense let, past participle læten), from Proto-Germanic *letan (cognates: Old Saxon latan, Old Frisian leta, Dutch laten, German lassen, Gothic letan "to leave, let"), from PIE *le- (2) "to let go, slacken" (cognates: Latin lassus "faint, weary," Lithuanian leisti "to let, to let loose;" see lenient). If that derivation is correct, the primary sense would be "let go through weariness, neglect."

Of blood, from late Old English. To let (something) slip originally (1520s) was a reference to hounds on a leash; figurative use from 1540s. To let (someone) off "allow to go unpunished" is from 1814. To let on "reveal, divulge" is from 1725; to let up "cease, stop" is from 1787. Let alone "not to mention" is from 1812.
let (n.)youdaoicibaDictYouDict
"stoppage, obstruction" (obsolete unless in legal contracts), late 12c., from archaic verb letten "to hinder," from Old English lettan "hinder, delay," from Proto-Germanic *latjan (cognates: Old Saxon lettian "to hinder," Old Norse letja "to hold back," Old High German lezzen "to stop, check," Gothic latjan "to hinder, make late," Old English læt "sluggish, slow, late"); see late.