This verb means to grow sharp or to sharpen, appearing only in Proverbs 27:17: 'Iron sharpens iron, and a man sharpens the countenance of his friend.' The saying uses the image of metalworking—one piece of iron hones another—to illustrate how people sharpen or refine each other through interaction. The exact parsing is debated (both verbs may be Hiphil, or one Qal and one Hiphil), but the sense is clear: mutual engagement produces sharpness, whether of blade or of character. It's a proverb of social formation and friendship.