Where does “tjugu” come from?

tjugu (Old Norse) comes from Old Norse tuttugu, from Proto-Germanic twai tigiwiz, from Proto-Germanic twai, from Proto-Indo-European dwóh₁ — two.

Ancestry of “tjugu”, step by step

Step	Language	Word	Meaning
1	Old Norse	tuttugu	twenty
2	Proto-Germanic	twai tigiwiz	twenty
3	Proto-Germanic	twai	two
4	Proto-Indo-European	dwóh₁	two

tjugu — Etymology, Origin & Word History | Etymology Explorer