A Gen whose fear response is greater than the usual fear of Simes. Much like people who have phobias of insects or animals. Sime phobia can be culturally induced (pre-Unity, out-Territory) or later it can afflict anyone after an experience, as happens with other phobias.
In Gens, phobia of any sort is almost a guaranteed death sentence in the junct era, since fear per se is the one Gen emotion that travels farthest and is most attractive to juncts. To the extent that "natural selection" has had time to operate, phobia itself is a malady bred out of Gens, though you might find it more pevalent among Simes.