straight, and so morally right: in line with the true standard, whether a road, a person, or God himself
The picture underneath this word is a line with no bend in it. A road can be yashar when it runs straight to where you are going, and a pair of legs or wings can be yashar when they stand straight rather than crooked. From that plain physical picture the Hebrew Bible draws its moral meaning: a person is yashar when their life runs straight, true to what is right, with no crookedness or double-dealing.
The same word names both the standard and the people who meet it. Often it appears in the phrase "what is right in the eyes of" someone. When the eyes are the LORD's, yashar names the real, fixed standard of right that he sets. When the eyes are a person's own, the same phrase can turn dangerous, describing whatever each person decides is right for himself, which the book of Judges treats as the root of chaos.
Its most common use is to describe character. "The upright" are a recognizable group in the Psalms and Proverbs, set against the wicked and standing alongside the righteous and the blameless. Strikingly, God himself is called yashar: he is upright, and so his ways and his rulings are straight, the very line by which everything else is measured.