no concern of mine; nothing to do with me
A set phrase expressing complete indifference or detachment, often used when someone refuses to get involved or feels unaffected by a situation. Literally 'I am not concerned'.
See also: 関する
He always acts as if it's none of his business.
Even when everyone around was making noise, she kept reading her book as if it had nothing to do with her.
From 我 (I) + 関せず (classical negative of 関する, 'to concern'). The phrase uses a classical negative form, giving it a somewhat literary or emphatic tone.