Not clear at all; confusing or incomprehensible, used ironically.
This ironic phrase emerged in the early 19th century American English, playing on the obvious contradiction between 'clear' and 'mud.' Mud is notoriously opaque and murky, making the comparison deliberately absurd. The humor lies in the complete opposition between the stated comparison and reality.
This phrase is a perfect example of ironic humor in language - it works precisely because mud is the opposite of clear, making the comparison laughably wrong. What's clever is how the phrase has become so established that people immediately understand the irony, showing how shared cultural knowledge allows us to communicate through deliberate contradictions.
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