This is the name that Arjuna, the great Pāṇḍava hero, assumed when he had to live in the palace of the king Virāṭa for one year as a eunuch, among the ladies of the palace, teaching them music and dancing.
When the Kauravas attacked Virāṭa’s capital and seized the palace cows, Bṛhannalā became the charioteer of Uttara-kumāra, the son of Virāṭa. Uttarakumāra went to the battlefield to repulse the attack but was too frightened to fight. So, Bṛhannalā had to reveal his identity and do the fighting himself.
Arjuna had once been cursed to become a eunuch, by the nymph Ūrvaśī since he had spurned, though respectfully, her amorous advances. The curse came in handy during the one year period the Pāṇḍavas had to live incognito as per the accord with the Kauravas, after they had been defeated in the game of dice.
See also ARJUNA.