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Phrase(s): *clean as a hound’s tooth [and] *clean as a whistle
1. Rur. Cliché very clean. (*Also: as ~.) • After his mother scrubbed him thoroughly, the baby was as clean as a hound’s tooth. • The car was as clean as a whistle after the Girl Scouts washed it. 2. Rur. Cliché innocent and free from sin or wrong. (*Also: as ~.) • Jane’s record was clean as a whistle; she had never committed even the smallest infraction.
McGrawhill's American Idioms And Phrasal Verbs