Wooden iron (German: hölzernes Eisen) is a polemical term often used in philosophical rhetoric to describe the impossibility of an opposing argument.
It occurs when a modifying adjective opposes its noun, as in "square circle," "freezing fire," "boiling snow," or "hard liquid."
For his argument against absolutes he wrote "... the categorical imperative leaps into the world, in order to command there with its unconditioned ought—a scepter of wooden iron."
Friedrich Nietzsche, who was an avid Schopenhauer scholar, continued to employ the term throughout his writings in the latter half of the 19th century.
The term was also employed in his poetic attempts to criticize socialism and the building of a "free society" (see Book 5 of The Gay Science, §356).