Being like a rock wall" is when a master of martial arts suddenly becomes like a rock wall, inaccessible to anything at all, immovable.
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About Miyamoto Musashi
Miyamoto Musashi was Japanese swordsman, philosopher, strategist, writer, artist, and rōnin (c. 1583–1645). Miyamoto Musashi was a Japanese swordsman, strategist, artist and writer who became renowned through stories of his unique double-bladed swordsmanship and undefeated record in his 62 duels. Musashi is considered a kensei of Japan. Read more on Wikipedia →