- It means to be confused or not understand something at all.
- A state of complete disorder.
- I was at sea when he started discussing quantum physics during dinner.
- When the teacher asked him a question about the subject, he was at sea.
- Even the judge was at sea listening to the unreasonable requests from the plaintiff and his lawyers.
- A medical professional sometimes gets a case that would put him at sea, at such time he would himself recommend a second opinion.
It comes from the times when navigation equipment was not available, and the seamen who ventured out were literally confused about which direction they were headed. Assumptions were made basis a lot of variants. While some of these assumptions were right, most were not. This would cause disorder in the journey and unnecessary delays. If a ship could not see any land, then there was a real danger of that ship being lost forever. The phrase has been in use since the 1700s, with the earliest literary reference in 1893 by Frederick C. Selous.