Pecs are the muscles on the front of your chest. This is the short, informal word people use in gyms and in everyday talk; the full word is pectorals. It is not rude, just casual. You almost always hear it in the plural: “nice pecs,” “work your pecs.” A single side can be “a pec,” but that is less common. Men talk about their pecs a lot when they lift weights; women have these muscles too, though people say the word less often about them.