Parody song is a comic song that has the same or very similar tune of a popular song but the words are changed. The earliest one I have heard is 'gaao gaao ai mere saadhoo' from MIss Frontier Mail (1936)
It is based on the very popular 'jaavo jaavo ai mere saadho' from Pooran Bhagat (1933)
Now you know!
Listen to Sati Nari (1965)