Many studies suggest that the brain processes swearing in the lower regions, along with emotion and instinct. Scientists theorize that instead of processing a swearword as a series of phonemes, or units of sound that must be combined to form a word, the brain stores swear words as whole units [ref]. So, the brain doesn't need the left hemisphere's help to process them. Swearing specifically involves:

* The limbic system, which also houses memory, emotion and basic behavior. The limbic system also seems to govern vocalizations in primates and other animals, and some researchers have interpreted some primate vocalizations as swearing.
* The basal ganglia, which play a large role in impulse control and motor functions.

So, you can think of swearing as a motor activity with an emotional component.