Noah Webster's New International Dictionary of the English Language

1. (v. t.) To annoy; to inflame; to render null and void.

2. (v. t.) To increase the action or violence of; to heighten excitement in; to intensify; to stimulate.

3. (v. t.) To excite anger or displeasure in; to provoke; to tease; to exasperate; to annoy; to vex; as, the insolence of a tyrant irritates his subjects.

4. (v. t.) To produce irritation in; to stimulate; to cause to contract. See Irritation, n., 2.

5. (n.) To make morbidly excitable, or oversensitive; to fret; as, the skin is irritated by friction; to irritate a wound by a coarse bandage.

6. (a.) Excited; heightened.