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.