It’s pronounced “here-eyeth” (roll the “r”) and it’s a Welsh word. It has no exact cognate in English. The best we can do is “homesickness,” but that’s like the difference between hardwood and laminate. Homesickness is hiraeth-lite. A quick history lesson is a good idea before a definition: in 1282 Wales became the first colony of the English empire. Because England eventually ruled half the globe, we all know its first colony by the name the colonizers gave it: Wales, which means “Place of the Others,” or “Place of the Romanized Foreigners.”
So that’s how the Welsh—the original Britons—became “foreigners” on their own island. Talk about a semantic insult.