Lukewarm
Here’s what it means
Spiritually complacent and useless rather than refreshingly beneficial.
Why the Bible says it this way
Jesus rejects a church whose works are neither helpfully hot nor refreshingly cold.
The writer uses a picture to explain one thing by describing it as another.
A helpful detail: Popular reconstructions of Laodicea's local water system are debated; the text clearly condemns useless complacency.