Roots of the word torrid are "torridus," which means scorching hot, and "torere," which means parched.
According to scholars, people in the 17th century began using the word torrid to describe objects that were very hot. A century earlier, people understood the term "torrid zone" to mean the tropics.
Today, people often use the adjective torrid to describe intensity of emotions. The cliche "a torrid love affair" is a good example, which describes an intense relationship. Another definition of torrid--hurried or fast-moving--augments this image of quick, hot intensity.
One synonym for torrid is "fervent," which contains a root word meaning "fire." A second synonym is "ardent." You can use either of these words to describe intensity of emotion.
Emily Dickinson chose to use the word in her poem, "A Shady Friend for Torrid Days," for its power to evoke both the image intense summer heat and intensity of emotion.