What is a "Naniwako"? Osaka's old name "Naniwa" and the nickname
Osakans are called "Naniwako" after the city's other name, Naniwa — a memory of a town long open to the sea.
"Naniwako" is an affectionate word for someone born and raised in Osaka.
The key is Naniwa, Osaka's old name — its origin, its spellings, and a poem that carries the name.
What "Naniwako" means
Naniwakonaniwako
"Naniwa" is an old name for Osaka; add "-ko" and you get "Naniwako," an affectionate word for someone born and raised there.
Source:コトバンク「難波（なにわ）」
Where "Naniwa" comes from (accounts vary)
Namihaya and Naniwa
The Nihon Shoki tells of "Namihaya-no-kuni," named for swift tides. Another account reads it as "naniwa," a place rich in fish. The origin is not settled.
Source:コトバンク「難波（なにわ）」
Spelling variants (難波 / 浪速 / 浪華 / 浪花)
難波 / 浪速 / 浪華 / 浪花
"Naniwa" has been written with several kanji. Note that 難波 is also read "Nanba" as a place name in the south of the city.
Source:goo国語辞書「難波／浪速／浪華／浪花（なにわ）」
The Naniwazu poem
The poem of Naniwazu
An old poem, "At Naniwazu the flowers bloom...", is well known; when Naniwa Ward was created in 1925, its name is said to have been taken from this verse.
Source:大阪市 浪速区 公式「区のおいたち」
FAQ
Are "Naniwa" and "Nanba" the same?
The kanji 難波 is usually read "Naniwa" for Osaka broadly, and "Nanba" for the district in the south. They come from the same word.
Is "Naniwako" different from "Osaka-kko"?
They mean much the same. "Naniwako" is simply a more evocative way to say it, using Osaka's old name Naniwa.