Tack, or cabin bread, is a food that's bitterly familiar to sailors; they're dense, hard, flavorless crackers made with just water and flour. It is miserable to eat and has very few nutrients, but it packs a lot of calories and can last for months, possibly even years. Tack is part of every sailor's rations. The lucky ones also have rum, and soaking a hard biscuit in rum can make it a lot more pleasant to eat and a lot easier to swallow.
Rumtack at the Low Lantern is a significant upgrade to a rum-soaked tack. Inspired by traditional black cake from the islands, this rich, boozy offering is one of the only food items available at the Lantern, and a good ship's captain will often order rumtack for the whole crew to help chase away the haunting memories of long months gumming at horrible crackers.