The idiomatic phrase by and large means
largely; generally; mostly
The two earliest usages listed in Google's ngram, from 1812 and 1837, appear to use it in its current form and meaning.
What is the origin of this phrase?
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by and large: it appears to have a nautical origin:
To get a sense of the original meaning of the phrase we need to understand the nautical terms 'by' and 'large'.
'Large' is easier, so we'll start there. When the wind is blowing from some compass point behind a ship's direction of travel then it is said to be 'large'*. Sailors have used this term for centuries; for example, this piece from Richard Hakluyt's The Principall Navigations, Voiages, and Discoveries of the English Nation, 1591: "When the wind came larger we waied anchor and set saile." When the wind is in that favourable 'large' direction the largest square sails may be set and the ship is able to travel in whatever downwind direction the captain sees fit.
'By' is a rather more difficult concept for landlubbers like me. *In simplified terms it means '*in the general direction of'. Sailors would say that to be 'by the wind' is to face into the wind or within six compass points of it.
The earliest known reference to 'by and large' in print is from Samuel Sturmy, in The Mariners Magazine, 1669.
By means into the wind, while large means with the wind. "By and large" is used to indicate all possible situations "the ship handles well both by and large".