If you're achy, your body hurts. A new boxer will undoubtedly feel achy after her first match.
When your feet are achy, they have a dull pain, possibly from wearing those brand new high heeled shoes. Some people feel achy all over when they get the flu, or after working out at the gym. The first use of the adjective achy was in a letter written by the novelist George Eliot in 1875, but its most well-known use is probably in country singer Billy Ray Cyrus's hit song "Achy Breaky Heart."