mod. very ugly. : This car's as ugly as sin, but it's cheap and dependable.
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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Physically or morally hideous, as in I can't think why she likes that dog; it's ugly as sin. This simile, first recorded in 1801, replaced the earlier ugly as the devil.
