"5촌 당숙" 당숙 is what you call your father's cousin, or "a paternal first cousin once removed". 촌 indicates the degree of kinship. Your parent is 2촌 , your uncle 3촌, and your cousin 4촌 (of these, only 삼촌 and 사촌 are common terms in daily life). If I have a 당숙, he is 4촌 (cousin) to my father, and therefore 5촌 to me. So "5촌 당숙" is a term referring to 당숙 with this 촌 detail. If you're interested the first chart on this page shows all relation names up to 8촌 on father's side, including 당숙: http://beintous.tistory.com/242.