to spare: to look on something with concern and hold back from harming or giving it up
This verb names a single motion that joins a feeling and an action: you see something, you are moved by it, and so you withhold the blow or refuse to let it go. Most often the Hebrew makes the eye the one who does it, as if the protective look itself is what stays the hand. Scholars argue over which side comes first, the inner pity or the outer sparing, but they agree the two are really one gesture. That is why the same word can describe holding back a sword, begging God to relent, and even hating to part with your belongings.
The senses below are shades of that one core, not separate words. When the object is a person in danger, the look spares a life (or is told not to). When the speaker pleads with God, the same look is being asked for as mercy. When the object is your own property, the look becomes reluctance to lose what is yours. The word travels easily between these because the underlying picture, the eye that cares enough to refrain, stays the same.
Its most famous home is a stern legal warning repeated in Deuteronomy: do not let your eye spare the offender. There the natural impulse to pity is named precisely so it can be overruled in the courtroom or under the ban of holy war. Later the prophets put that very warning into God's own mouth against a guilty Israel, which makes its reappearance as a plea (Joel, Nehemiah) and finally as God's question over Nineveh in Jonah all the more striking: the eye that was commanded not to spare turns out to be the eye that, in the end, spares.