Nadako is possibly the mistaken joining of the two words na and dako when they occur adjacent in a sentence.
There is also use of it in an uncommon word formation for dako.
Kailan nga ba sa buhay mo, ika’y nadako sa aklatan?
When in your life have you been towards the library?
Further, it could be a word from another Philippine language distinct from Tagalog. The word formation suggests one of the Visayan languages like Waray.
An tawo nga diri maupay
An pag-ataman nadako
Nga waray batasan
