Anyone know what this brown line growing on my fingernail could be?

Melanonychia. Longitudinal melanonychia may be a sign of subungual melanoma (acral lentiginous melanoma), although there are other diagnosis such as chronic paronychia, onychomycosis, subungual hematoma, pyogenic granuloma, glomus tumour, subungual verruca, mucous cyst, subungual fibroma, keratoacanthoma, carcinoma of the nail bed, and subungual exostosis.