Lacto ring Share definition Flag The plastic circle that stays attached to the gallon milk jug when the cap is removed. Makes an ideal cat toy. I cleaned under the fridge yesterday and found 3 quarters, 2 pens and 9 lacto rings. Lacto ring by Gourdaholic May 17, 2010 0 0 Get merch Get the Lacto ring mug. Buy Now — $32.95 See more merch