Transit means “journey,” and saying you’re "in transit" means you’re on your way somewhere. Before we had cell phones, one of the upsides of being in transit was that no one could get in touch with you.
Transit may be familiar to you from the phrase mass transit. While it sometimes seems like “mass transit” is synonymous with “unreliable, crowded and inefficient,” the phrase just means "public transportation." The first instance of transit being used in this sense was in the 19th century, and people have been complaining ever since. Transit is related to the word transition, which refers to a change from one state or condition to another.