it's none of one's business; it's not one's place to say or do something
Used to indicate that the speaker has no right or reason to interfere, speak, or act in a given situation. Often used humbly or to dismiss someone's involvement.
彼の家庭の問題だから、私が出る幕ではない。
It's his family matter, so it's not my place to interfere.
専門家の前で素人が意見を言うなんて
It's not my place as an amateur to give an opinion in front of experts.
From 出る (to appear) + 幕 (curtain, act in a play) + ではない (is not). Literally 'it is not the act/scene where one appears', i.e., it's not one's turn to come on stage, hence 'it's not one's place to interfere'.