This was the ultimate mislead. I was able to figure out what every word is supposed to mean except this one, and it was driving me crazy. I eventually looked up the ingredients:

The Big Mac is made with two 1.6 oz (45 g) beef patties, a “special sauce” (similar to Thousand Island dressing),[7] shredded iceberg lettuce, one processed American cheese slice, two slices of dill pickle, and minced onions, served on a three slice sesame seed bun.[8]

Only the cheese, sesame seeds, and I guess maybe the seasoning are missing. Is a seed a sack? I am so unsatisfied.