From Old Norse mjǫlk, mjolk, from Proto-Germanic *meluks, cognate with Danish mælk, Swedish mjölk, English milk. Ultimately from Proto-Indo-European *h₂melǵ-. Note the lack of i-mutation in the genitive (from *melukiz). Although the form would suggest a mutated **mýlkur, -jó- is not the original vowel (from *-eu-, as in þjóð), but a product of a double vowel change, first fracture: meluk- > mjǫlk, and then lengthening: o/ǫ > ó before lk. Also, the ability of the -iz to effect an i-mutation on the first vowel is reduced by the intermediate -u-.