There are numerous synonyms for the phrase "hard to explain." Some of them include complicated, intricate, perplexing, enigmatic, elusive, insoluble, inscrutable, mystifying, obscure, and unfathomable. These words all describe something that is difficult to understand or describe in simple terms. Whether it's a complicated mathematical concept that is hard to express in layman's terms or a mysterious occurrence that is difficult to explain, these synonyms can be used to convey the same sense of complexity and difficulty to articulate. Ultimately, the choice of synonym will depend on the context and the specific nuances of the situation being described.