Immovable means unmoving.
Wisdom means the wisdom of intelligence.
Although wisdom is called immovable, this does not signify any insentient thing, like wood or stone. It moves as the mind is wont to move; forward or back, to the left, to the right, in the ten directions and to the eight points; and the mind that does not stop at all is called immovable wisdom.
The Unfettered Mind: Writings of the Zen Master to the Sword Master, Takuan Soho, p. 20