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what does "J'en ai à faire" mean?

It means "I have to make it."

"Avoir de qch a faire" means "have sth to do" . When "avoir" is used after "je" , it changes into "ai", and "de+ qch" change into "en" when they are put in front of the verb. "qch" means something.

j'en ai rien à faire = cela ne m'interesse pas

It means 'it looks at me', 'it's my business'

The most natural answer for me as a Frenchman would be the same as Alexis. Most common is the negative form of this expression "je n'en ai rien à faire" meaning "i really don't care", "it doesn't interest me". It doesn't mean then "i have something to do" which would be translated by "j'ai quelque chose à faire" (without "en").