Dafa kar is a song composed and sung by A R Rahman for the film Heropanti 2. The song is written by Mehboob Kotwal.
'dafa kar' or 'dafaa kar' (दफ़ा कर) is an interesting choice of words which uses two of the meanings of the word 'dafa'. One of the meanings of 'dafa' is to ward something off. It's used as 'dafa hona' or 'dafa karna'. So when you say 'dafa ho' to someone, you ask them to just go away, and if you say 'isey dafa kar' for something, you are asking them to get rid of it.
Another meaning of 'dafa' is times or occasions. That is, if you do something twice, you could say you did it 'do dafa'. So when you say 'sau dafa kar', you literally mean 'do (it) a hundred times'.
Thus, the first line, 'dil jo bole, sau dafa kar' means, 'What your heart tells you, do it a hundred times.'
While the second line, 'koi na maane, dafa kar' means, 'if someone doesn't listen to you, get rid of them.'
Oh, a probably unintentional meaning is there too, since 'dafa kar' sounds quite a bit like English 'the fu**er' as well. But I don't really think that's intentional here.