stand-by n.

Old English standan "occupy a place; stand firm; congeal; stay, continue, abide; be valid, be, exist, take place; oppose, resist attack; stand up, be on one's feet; consist, amount to" (class VI strong verb; past tense stod, past participle standen), from Proto-Germanic *standanan (source also of Old Norse standa, Old Saxon and Gothic standan, Old High German stantan, parallel with simpler forms, such as Swedish stå, Dutch staan, German stehen [see discussion in OED]), from *stathula, from PIE root *sta- "to stand, make or be firm."

Sense of "to exist, be present" is attested from c. 1300. Meaning "encounter without flinching" is from 1590s; weaker sense of "put up with" is from 1620s. Meaning "to submit" (to chances, etc.) is from c. 1700. Meaning "to pay for as a treat" is from 1821. Meaning "become a candidate for office" is from 1550s. Nautical sense of "hold a course at sea" is from 1620s. Meaning "to be so high when standing" is from 1831.

Stand back "keep (one's) distance" is from c. 1400. Phrase stand pat is from poker (1882), earlier simply stand (1824 in other card games). To stand down is from 1680s, originally of witnesses in court; in the military sense of "come off duty" it is first recorded 1916. To let (something) stand is from c. 1200. To stand for is c. 1300 as "count for;" early 14c. as "be considered in lieu of;" late 14c. as "represent by way of sign;" sense of "tolerate" first recorded 1620s. Phrase stands to reason (1620) is from earlier stands (is constant) with reason.

Old English be- (unstressed) or bi (stressed) "near, in, by, during, about," from Proto-Germanic *bi "around, about," in compounds often merely intensive (source also of Old Saxon and Old Frisian bi "by, near," Middle Dutch bie, Dutch bij, German bei "by, at, near," Gothic bi "about"), from PIE *bhi, reduced form of root *ambhi- "around."

As an adverb by c. 1300, "near, close at hand."

OED (2nd ed. print) has 38 distinct definitions of it as a preposition. Originally an adverbial particle of place, which sense survives in place names (Whitby, Grimsby, etc., also compare rudesby). Elliptical use for "secondary course" was in Old English (opposed to main, as in byway, also compare by-blow "illegitimate child," 1590s, Middle English loteby "a concubine," from obsolete lote "to lurk, lie hidden"). This also is the sense of the second by in the phrase by the by (1610s).

By the way literally means "along the way" (c. 1200), hence "in passing by," used figuratively to introduce a tangential observation ("incidentally") by 1540s. To swear by something or someone is in Old English, perhaps originally "in the presence of." Phrase by and by (early 14c.) originally meant "one by one," with by apparently denoting succession; modern sense of "before long" is from 1520s.

By and large "in all its length and breadth" (1660s) originally was nautical, "sailing to the wind and off it," hence "in one direction then another;" from nautical expression large wind, one that crosses the ship's line in a favorable direction.