Definition: Giving opinions outside one’s knowledge or area of expertise (adjective or noun).

Thinking he was attending the annual Astronomy Society dinner, the noted astronomer accidentally wandered into a meeting of the Astrology Association across the road.

The attendees were learning how to read their future in the stars.

Unfortunately, the constellation they selected had imploded several million years ago. When the astronomer pointed this out and questioned its relevance to the future, he was told that ultracrepidarians were not welcome and ejected from the meeting.