Where does “मार्गशीर्ष” come from?

मार्गशीर्ष (Sanskrit) comes from Pali Māgasira.

Ancestry of “मार्गशीर्ष”, step by step

Step	Language	Word	Meaning
1	Pali	Māgasira	—

मार्गशीर्ष — Etymology, Origin & Word History | Etymology Explorer