It describes the awkwardness when someone walks one way and someone else walks another (so that they're walking towards each other) and the two people both step the same way so that the other person can pass. However, they end up moving from side to side at the same time... it's rather embarrassing.

A portmanteau of carpe diem, a Latin phrase that means “Seize the Day!” and DM, which stands for Direct Messages. This word means seizing the day via DMs.