。
When unreason passes, it becomes reason.
Literal
Unreason [also-も] pass [conditional-ば] reason [become-となる].
A proverb about the corrupting power of accepted injustice. 無理 ('unreason, impossibility') + も adds 'even.' ~となる is the formal 'become X' pattern. The full idea: 'when the unreasonable is accepted, it passes for the reasonable.' A counterpart to the more famous 道理が引っ込む ('reason retreats').