letteryoudaoicibaDictYouDict[letter 词源字典]
letter: [13] The distant ancestry of the word letter has never been satisfactorily explained. One possible candidate as a source that has been put forward is Greek diphthérai ‘writing tablets’. But the earliest precursor that can be positively identified is Latin littera. This meant ‘alphabetic symbol’, or in the plural ‘document’ and ‘epistle’. English acquired it via Old French lettre. Also from Latin littera are English literature and obliterate [16], which means etymologically ‘remove letters’.
=> literature, obliterate[letter etymology, letter origin, 英语词源]
letter (n.1)youdaoicibaDictYouDict
c. 1200, "graphic symbol, alphabetic sign, written character," from Old French letre (10c., Modern French lettre) "character, letter; missive, note," in plural, "literature, writing, learning," from Latin littera (also litera) "letter of the alphabet," of uncertain origin, perhaps via Etruscan from Greek diphthera "tablet," with change of d- to l- as in lachrymose. In this sense it replaced Old English bocstæf, literally "book staff" (compare German Buchstabe "letter, character," from Old High German buohstab, from Proto-Germanic *bok-staba-m).

Latin littera also meant "a writing, document, record," and in plural litteræ "a letter, epistle," a sense first attested in English early 13c., replacing Old English ærendgewrit, literally "errand-writing." The Latin plural also meant "literature, books," and figuratively "learning, liberal education, schooling" (see letters). School letter in sports, attested by 1908, were said to have been first awarded by University of Chicago football coach Amos Alonzo Stagg. Expression to the letter "precisely" is from 1520s (earlier as after the letter). Letter-perfect is from 1845, originally in theater jargon, in reference to an actor knowing the lines exactly. Letter-press, in reference to matter printed from relief surfaces, is from 1840.
letter (v.)youdaoicibaDictYouDict
"to write in letters," 1660s, from letter (n.1). Earlier it meant "to instruct" (mid-15c.). Related: Lettered; lettering.
letter (n.2)youdaoicibaDictYouDict
"one who lets" in any sense, c. 1400, agent noun from let (v.).