Horses' ages and quality were checked by looking at their teeth. Pigs have a very bad reputation for being gross hedonists, disgusting, disagreeable, grumpy, unclean, ugly, and so on. This may be unfair to pigs, but it's very much part of the language. If you're eating like a pig , you're eating a lot, quickly, and not caring much what it is you're eating. Pigs live in a pig sty , and if people say you do, too, it means your home is messy, if not filthy.
Pigs eat unappealing food called slop .
If a person is a pig , or piggish , they are either sloppy, ugly, stubborn, or cruel - or a police officer. For unknown reasons, pig is the standard insult for police. Other references may be made to bacon or pork . Police officers hate this very much, and they will resent such talk. They usually won't mind the casual, but not insulting cop .