janmana janakah so ’bhud
vaidehas tu videhajah
mithilo mathanaj jato
mithila yena nirmita
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Because he was born in an unusual way, the son was called Janaka, and because he was born from the dead body of his father, he was known as Vaideha. Because he was born from the churning of his father’s material body, he was known as Mithila, and because he constructed a city as King Mithila, the city was called Mithila.