Sick to one's stomach

What's the meaning of the phrase,"sick at (or to) one's stomach?" I would be very pleased if someone could also tell me, how did the phrase originate?

The phrase generally means "feeling nauseous." It reflects the bodily source or location of the feeling, as interpreted by our brain. The feeling of nausea isn't necessarily caused by an actual condition of the stomach. (It is often caused by a smell, for instance, or by heavy drinking.) But it inevitably causes sensations in the stomach area.
SS