Thumb Soup™ /Thum∙Soop/ Noun.
A little special something a customer gets when ordering Pho at a Vietnamese noodle shop. Typically occurs when a waiter brings out a bowl of steaming hot noodles without the aid of a tray. Cradling the bowl with four fingers to carrying the weight of the bowl requires the use of the waiter’s remaining finger, the thumb, for balance. This inevitably leads to the thumb being dipped into the soup, adding a little extra flavor to the soup. Now that you know the secret, you will now, of course, stare directly at your bowl of soup when it comes out to see at which point (if ever) it instantly transforms to Thumb Soup™.
Term was originally coined by Brian M.