clear是清楚明白的意思，但是这个表达并不是一清二楚的意思。恰好相反，因为mud是泥土，所以as clear as mud的意思是难懂的、乱七八糟、不清晰、混乱的意思。

【例句】
To be honest,your handwriting is really as clear as mud.
说实话，你的笔迹真的很难看懂。

Your report was badly written, it was as clear as mud.
你的报告写得很糟，简直看不懂。