Complex sentence: The term "slush fund" was originally a nautical term: the slush was the fat or grease that was obtained by boiling salted meat, the sale of which could then be used to provide the crew with special luxuries.
Split into simpler sentences: The term "slush fund" was originally a nautical term: the slush was the fat or grease that was obtained by boiling salted meat. The sale of the fat could then be used to provide the crew with special luxuries.