In FFXIV, the Prime Loaghtan Rump is a crafting material used to create certain food items as a Culinarian crafting class. It can be purchased from any Scrip Exchange NPC (such as the one in the Studium in Old Sharlayan or in down Hawker’s Alley Limsa Lominsa). Each piece of meat costs 15 Purple Crafters’ Scrips apiece.
The Culinarian recipes that require it also require the player to first purchase (and use) a copy of Master Culinarian X — which can also be purchased at the Scrip Exchange for White Crafters’ Scrips.
Prime Loaghtan Rump was added to the game in Patch 6.2 during Endwalker.