From Straightdope:
What is the origin of the phrase “by and large?” Why do Americans say something so completely inane? My sister has been agonizing over that one for years. (Well, maybe she hasn’t actually agonized over it, but she really wants to know.) —Robert Cook
SDSTAFF Dex replies:
Americans? Olivia Isil cites the BBC’s highly acclaimed screen adaptation of Jane Austen’s Pride and Prejudice, where Miss Bennet remarks on the betrothal of Mr. Wickham to the odious Miss King: “by and large, it was to be expected.”
The phrase “by and large” today means “generally speaking,” “mostly” or “on the whole.” The origin is nautical, and had a very precise meaning. It was an order to the man at the helm of a sailing ship, meaning to sail the ship slightly off the wind. A similar command was “full and by” which meant to “sail as close to the wind as it can go.”
The risk of sailing too close to the wind was the danger of being “taken aback” (when the sails press against the mast and progress halts.)
Thus, when a person doesn’t want to “sail” directly into a statement, “by and large” is a hedge, a phrase of circumspection, a way of saying that the statement is an imprecise generality.
Sources:
Olivia Isil, When a Loose Cannon Flogs a Dead Horse, There’s the Devil to Pay (seafaring words in everyday speech)
William and Mary Morris, Morris Dictionary of Word and Phrase Origins
Brewer’s Dictionary of Phrase and Fable
Thanks for that, a, tidbit.