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.
American Heritage Idioms
Phrase(s): *ugly as sin
Cliché extremely ugly. (*Also: as ~.) • Why would anyone want to buy that dress? It’s as ugly as sin! • Harold is ugly as sin, but his personality is very charming.
McGrawhill's American Idioms And Phrasal Verbs