straight: running without bends or detours, whether it is a road or a life
The picture behind the word is a line with nothing bent in it. A straight road is the one that gets you there without a detour, and in the ancient world roads really were repaired and straightened ahead of a visiting king. That is the scene Isaiah paints and the first three Gospels quote: fill in the valleys, level the hills, take the kinks out of the road, because someone important is coming.
From roads the word moves easily to people, because a life is often pictured as a road. A straight heart, or a straight way, is one that goes where it says it is going, with no doubling back and no hidden turns. Peter tells Simon in Samaria that his heart is not straight before God, and Paul tells a magician on Cyprus that he has been putting bends in the straight roads of the Lord. English usually switches to "right" or "upright" at that point, which is accurate, though the road quietly disappears.
Two smaller notes. In Acts 9:11 the word has hardened into a place name: the street in Damascus where Paul was staying was simply called Straight. And the noun built from the same root (εὐθύτης) names the quality on its own, uprightness, which is what Hebrews 1:8 says the scepter of God's kingdom is made of.
A different Greek word (ὀρθός) covers standing upright and keeping a path level underfoot. This one is specifically about the absence of bends.