It’s a concept in Japanese martial arts that I read in a book by Takuan Soho.

Wiki:

Fudōshin (Japanese: 不動心) is a state of equanimity or imperturbability (literally and metaphorically, “immovable mind”,[1] “immovable heart”[2] or “unmoving heart”). It is a philosophical or mental dimension to Japanese martial arts which contributes to the effectiveness of the advanced practitioner.

I was training in Jujutsu and kenjutsu at the time and the concept clicked. I know it sounds very ‘weeb’ but it was an epiphany and quite profound for me at the time.

There’s a Shingon Buddhist deity that encapsulates the concept too and he holds a sword to cut through ignorance and delusion.

There’re some related concepts like Mushin that come together with marital arts.

• Fudoshin - https://en.m.wikipedia.org/wiki/Fudōshin
• Takuan Soho - https://en.wikipedia.org/wiki/Takuan_Sōhō
• Mushin - https://en.m.wikipedia.org/wiki/Mushin_(mental_state)