Origin and history of breadth

Old English lengðu "property of being long or extended in one direction; distance along a line," from Proto-Germanic *langitho, abstract noun from *langaz "long" (root of Old English lang; see long (adj.)) + *-itho, abstract noun suffix (see -th (2)). Cognate with Old Norse lengd, Old Frisian lengethe, Dutch lengte.

Figurative sense of "the distance one goes, extremity to which something is carried" is from 1690s. Phrase at length "to full extent" is attested from c. 1500. As "the length of a swimming pool," 1903. From the notion of "a piece or portion of the extent of anything" come the theater slang sense "a 42-line portion of an actor's part" (1736) and the sporting sense "the length of a horse, car, etc. in a race" used as a unit of measure (1650s).

also hairsbreadth, hairs-breadth, hair's breadth, from late 15c. as a measure of minute exactness. It is said to once have been a formal unit of measure equal to one-forty-eighth of an inch. From hair + breadth.

"state or character of being wide; lineal extent of a thing from side to side;" 1620s, formed from wide on model of breadth, and displacing wideness. Johnson (1755) calls it "a low word." OED (1989) characterizes it as "a literary formation." Related: Widthwise.