by and large
On a sailing ship or sailboat, new sailors at the helm (steering wheel) are usually ordered to sail "by and large." This means that they are to sail into the wind at an angle, as opposed to directly into the wind. Sailing directly (or almost directly) into the wind is most efficient, however doing so may cause the sail to flap back against the mast (the pole supporting the sail), resulting in loss of speed and control.
Sailing "by and large" thus means to set a course that is generally correct, if not exact. Over time, the phrase has become a synonym for "approximately."