To discomfit someone is to make them feel uncomfortable or upset. An easy way to discomfit another person is to use the age-old, childish trick of ignoring them. (Of course, we’re sure you would never do that, right? Right?)
Discomfit looks and sounds a lot like discomfort, which would seem to make sense given that these words are so similar in meaning. However, they were not always so closely linked. Centuries ago, discomfit was used to mean “destroy completely in battle” — an experience that surely left the defeated armies feeling something more than mere discomfort. Some scholars have suggested that the contemporary meaning of discomfit arose due to confusion with the word discomfort.