Yaaph (H3287) describes exhaustion, weariness, or faintness — the depletion that comes from hard travel, labor, or prolonged trial. It often portrays the human state in contrast to God's inexhaustible strength.
Isaiah 40:28–31 is the great reversal of yaaph: the everlasting God does not grow weary; He gives strength to the weary. Those who wait on the LORD will soar on wings like eagles — they will run and not grow weary, walk and not be faint. This word defines the human condition that divine grace meets and overcomes.