To speak quietly enough so as not to be heard, yet loudly enough so as to make the people on the adjacent table aware they are being complained about.
Not to be confused with "shoomble", which refers to black markets exclusively trading in niche items.
Ex. "I swear they stole our wine menus," she shumbled from Table 7. "I'm going to steal them back when they're not looking."
"I swear they stole our wine menus," he shumbled from Table 5. "That's why I stole them back when they weren't looking."
"Fools", shumbled the waiter from the foyer. "With all these wine menus, I could start a shoomble."