Arrows, sides of a rectangle, well-organized lines of people waiting to get into the theater or the ladies' room, and anything else free from curves can be described as straight. The word can also mean honest, uninterrupted, undiluted, or heterosexual.
In Middle English, strecchen meant "to stretch." Streght "stretched" emerged from it and morphed into straight. It's easy to see the connection: If you stretch out a piece of spaghetti or a curl, it looks linear. Today the word means free from bends, breaks, or irregularities literally or figuratively. "Give it to me straight" means "just the facts, ma'am." "I was up for seven straight days" means "I haven't slept in a week."