someone who vomits as an instant reaction to someone else vomiting. the adjective 'sympathy' is used not because the second vomiter feels bad for the first, but be cause they cannot control that vomit reflex when they see someone else vomit.
Today, I was impressing dinner guests by spinning my new baby in the air (something she loves), when she projectile vomited over the dinner table and the guests. My wife, who had spent three hours cooking was not impressed. Once of the guests was also a sympathy spewer. FML