The word 无坚不摧 is formed by sequentially combining the characters 无 (without), 坚 (hardness), 不 (not), and 摧 (to destroy) into a four-character structure where 无 negates 坚 and 不 negates 摧, creating a literal meaning of "there is no hardness that cannot be destroyed", which functions as an idiom expressing the idea of invincible force.