From Norman caillou, of uncertain origin. Compare Picard cailleu, Walloon caie. Perhaps from Dutch kei (“stone, rock”) or Gaulish cala.
caillou m (plural cailloux)
- gravel, small stone
Only seven words in French ending with -ou have their plurals in -oux instead of -ous: bijou, caillou, chou, genou, hibou, joujou, pou.