Where does “सेना” come from?

सेना (Marathi) comes from Sanskrit सेना, from Proto-Indo-Iranian sáynaH — army, troop; horde, band.

सेना (Marathi): army

Definitions

Step	Language	Word	Meaning
1	Sanskrit	सेना	an army, armament, battle-array, armed force;...
2	Proto-Indo-Iranian	sáynaH	army, troop; horde, band

सेना — Etymology, Origin & Word History | Etymology Explorer