SB 9.13.13

janmanā janakaḥ so ’bhūd
 vaidehas tu videhajaḥ
mithilo mathanāj jāto
 mithilā yena nirmitā
Word for word: 
janmanā — by birth; janakaḥ — born uncommonly, not by the usual process; saḥ — he; abhūt — became; vaidehaḥ — also known as Vaideha; tu — but; videha-jaḥ — because of being born from the body of Mahārāja Nimi, who had left his material body; mithilaḥ — he also became known as Mithila; mathanāt — because of being born from the churning of his father’s body; jātaḥ — thus born; mithilā — the kingdom called Mithilā; yena — by whom (Janaka); nirmitā — was constructed.
Translation: 
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 Mithilā.