No idea of the origin but to be "sent to Coventry" means no one speaks to you or acknowleges your presence. It's my understanding that it is common in English public (US private) schools. The practice was indulged in in school I attended but teachers put immediate stop to it saying it was too cruel for the minor social offenses perpetrated by grammar school children. Here are common bits of animal-based slang off the top of my head: scarce as hen's teeth; sick as a dog; raining cats and dogs; lower than a snake's belly; horny as a two-peckered billy goat. A bit scatalogical but succinct. Sorry if anyone finds it offensive. It is a very old saying. WOW