Where does “ניסן” come from?

ניסן (Aramaic) comes from Akkadian nisannum, from Sumerian 𒉠 — first-fruit offering.

Ancestry of “ניסן”, step by step

Step	Language	Word	Meaning
1	Akkadian	nisannum	offering of first produce of the season, offering of first fruits
2	Sumerian	𒉠	first-fruit offering