draw the line at something

Example: “I'll help you move, but I draw the line at carrying the piano.”

Origin: The origin is genuinely unsettled, with candidates including boundary furrows plowed to mark the edge of a property, the marked limits of early court games, and the scratched line at the center of a prize ring that fighters had to come up to — the same line that gave English "toe the line" and "up to scratch." What all of them share is the property that makes the metaphor work, and it is a slightly paradoxical one: a drawn line is arbitrary in its placement, since it could always have been drawn a little further one way or the other, but absolute in its function once drawn, admitting no degrees. That is exactly the character of a stated principle, which is why the phrase has outlasted certainty about where it came from.