byyoudaoicibaDictYouDict[by 词源字典]
by: [OE] By comes from a prehistoric Germanic *bi, which appears ultimately to be the same form as the second syllable of Latin ambi- (as in ambidextrous), Greek amphí (as in amphitheatre), and Old English ymbe, all of which meant ‘on both sides, round’. The original meaning of by thus seems to be ‘close to, near’. By is the basis of the prefix be-, as in befall and belong.
[by etymology, by origin, 英语词源]
by (prep.)youdaoicibaDictYouDict
Old English be- (unstressed) or bi (stressed) "near, in, by, during, about," from Proto-Germanic *bi "around, about" (cognates: Old Saxon and Old Frisian bi "by near," Middle Dutch bie, Dutch bij, German bei "by, at, near," Gothic bi "about"), from *umbi (cognate with second element in PIE *ambhi "around;" see ambi-).

Originally an adverbial particle of place, in which sense it is retained in place names (Whitby, Grimsby, etc.). Elliptical use for "secondary course" (opposed to main, as in byway, also compare by-blow "illegitimate child," 1590s) was in Old English. This also is the sense of the second by in the phrase by the by (1610s). By the way literally means "in passing by" (mid-14c.); used figuratively to introduce a tangential observation by 1540s.

Phrase by and by (early 14c.) originally meant "one by one," modern sense is from 1520s. By and large (1660s) originally was nautical, "sailing to the wind and off it," hence "in one direction then another."