c.1590, alteration of heyda (1526), exclamation of playfulness or surprise, something like Mod.Eng. hurrah, apparently an extended form of M.E. interjection hey, hei. Modern sense of "stage of greatest vigor" first recorded 1751, which altered the spelling on model of day, with which this word apparently has no etymological connection.
Build a man a fire and he's warm for a day. Set a man on fire and he's warm for the rest of his life.
