Janmejay was a Hindu King of the Pandava Dynasty, Great Grandson of Arjun, Abhimanyu's grandson, Son of King Parikshit of the Mahābhārata fame. Known as the historical figure who did a nag yagya and killed all snakes to seek revenge of his father's death due to a snake bite. Janmejay is also known as the first ruler of the era of Kalyug or the era of 'Kali'.
The meaning of janmejay is "he who uplifts the people" or "he who makes people tremble". The derivation is from the noun "jan" (meaning people like in Janata) and the "ij" verb which gets converted into "ejay".
Literally speaking, in Hindi, Janmejay gets translated as a compound word, with 'Janm' meaning 'birth', and 'Ajay' meaning 'invincible'. Thus, the word translates to 'invincible from birth' or the 'one who cannot be conquered since birth'.
Janmejay is known for the famous Nag Yagya. He wanted to avenge the death of his father Parikshit and kill the serpent king Takshak and when he could not find him since he was being protected by Lord Indra, he performed Nag Yagya and killed all snakes from the face of earth. Most likely Takshak was a king of Agni-Kula who were arch rivals to the rulers of Hastinapur (now Delhi) who ruled from what is now Ajmer.