From the Dune Wiki:
The term originated from the Bene Gesserit's prescient powers, and their inability to venture into a specific region of prescient knowledge. This region, though mysterious in nature, was known to be unattainable to females. Specifically, the spice melange allowed the Bene Gesserit to unlock genetic memory, but only on their maternal side. Female humans carry two X chromosomes, while males possess an X chromosome and a Y chromosome. A Kwisatz Haderach would be capable of accessing genetic memories stored within both chromosomes, while those memories were inaccessible to Bene Gesserit. Memories from male ancestors are still accessible to females with access to genetic memory (Alia is able to make contact with the ego memory of Vladimir Harkonnen for instance) but due to the lack of the Y chromosome, the memories were incomplete. Accessing the full paternal memories was impossible for the Bene Gesserit, and the very thought of trying was terrifying to them. Further, melange also exposed the Sisterhood (and others) to a limited form of prescience - thoughts, feelings, images into the near future, but no more.