perish the thought

Perish the thought!

1. said to show that you hope that something that has been suggested will never happen — и думать забудь; даже не думай; выкини это из головы! ~отдохни от этой мысли; да типун тебе на язык!

Example: Me, get married? Perish the thought!

2. something that you say which means you hope very much that something does not happen — не дай бог! боже упаси!, (как междометная вставка)

Example: If his father came to live with us, perish the thought, I can't imagine what strain that would put on our relationship. (СIDI)

Related vocabulary:
put sth out of one's mind (to not think about someone or something)
— выкинуть, выбросить из головы; перестать думать о чем-л.; избавиться от какой-л. (навязчивой или неприятной) мысли

Example: I tried hard to put the conversation with my mom out of my mind.(Cambridge Academic Content Dictionary)

cf.
[out of one's system]