Hot is hot and cold is cold, and in the middle is “warm.” If you’re talking about certain things (a day or a slice of toast for example), “warm” is a word you’d be likely to use. But other things (for some reason, liquids; water or milk) you might use “lukewarm” even though they could be exactly the same temperature as what you call “warm”.
“Lukewarm” is put together out of “warm” and, obviously, “luke.” “Luke” is derived from the Old English “hleowe,” which became “lew” in English, and finally assumed its current form. It’s only used today in “lukewarm” but when it was a standalone word all three versions of “luke” meant, well, “warm.” So when you say “lukewarm” you’re in a sense saying “warm warm.”
If you’d prefer to stick to Latin-derived words you could say “tepid” instead. “Tepid” means exactly the same thing as both “warm” and “lukewarm,” and comes from the Latin word “tepidus.” You already know what that means, right? Both “tepid” and “lukewarm” showed up in English between the 1300s and 1400s. They meant exactly the same thing back then, too, and both of them had acquired figurative meanings by the early 1500s. If a person is only mildly interested in something, they might be called “lukewarm” on the subject. They might also be called “tepid” — the figurative meanings of the two words, just like the literal meanings, are exactly the same.
One thing that’s almost never addressed by researchers who study word origins and linguistic connections is why, sometimes, words with identical usage seem to appear at about the same time, and are both used. You might expect the meanings to diverge to some extent, which they often do. But in this case English speakers have a choice of two words that have for centuries been virtually interchangeable. No idea why; I guess everybody has always been just kind of lukewarm on the idea of figuring it out.
