The story
Refers to the district of Nilphamari, which is a transliteration of নিলফামারী (nilphamari), a corruption of নীলখামারী (nilkhamari), a compound of নীল (nil, “indigo”) + খামারী (khamari, a suffixed form of খামার (khamar, “farm”)), literally meaning "indigo farm", referring to the former indigo cultivation in this region.
Etymology from Wiktionary, CC BY-SA 4.0.