Where does “lunis” come from?

lunis (Sardinian) comes from Latin Lūnae dīēs, from Latin dies Lūnae, from Ancient Greek σελήνη, from Ancient Greek Σέλας.

Ancestry of “lunis”, step by step

Step	Language	Word	Meaning
1	Latin	Lūnae dīēs	—
2	Latin	dies Lūnae	—
3	Ancient Greek	σελήνη	moon; month; a moon-shaped cake
4	Ancient Greek	Σέλας	—