Je m'en bats les couilles

Translated directly, je m'en bats les couilles means something like 'I beat my balls with it'.

Couilles is French for 'balls' – as in the male genitalia and not the ones used for football – and battre means 'to punch'.

However s'en battre les couilles (beating one's balls with something) is not a referral to some (very) odd French sport – it's simply a way to express indifference.

There are lots of ways to say you don't care in French, and je m'en bats les couilles is definitely one of the most vulgar ones.

The closest English equivalent is probably 'I don't give a fuck'.