LORELAI: Hey, four menus, a coffee and an anvil please.
LUKE: What’s the anvil for?
LORELAI: For Rune.
Lorelai is referring to a common trope in cartoons where an anvil is dropped on a character’s head with hilarious results. It seems to have first been used in Disney animated films, and was perfected by Warner Bros. in their Looney Toons cartoons. (Possibly not a coincidence that Lorelai sees Rune off with a “Bye, Loon”.
The comedy anvil drop may have its origins in real life. A traditional celebration on the Fourth of July in America was launching an anvil into the air with gunpowder from atop another anvil, then watching it fall onto the other anvil with a thud. Presumably everyone stood well back during this exciting spectacle and hopefully nobody got an anvil on the head.