Srimad Bhagavatam Canto 09, Chapter 13, Text 13

SB 9.13.13

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.
Srimad Bhagavatam Canto 09, Chapter 13, Text 12
Srimad Bhagavatam Canto 09, Chapter 13, Text 14