It’s great that you can go through life using a phrase for 50 years and not realise that it’s actually one word. I’ve no idea why I never realised (or have forgotten?) that “lukewarm” is one word. It is not “luke warm” as I thought. But, apparently, it’s just an old word and not two words. I wanted to write “luke warm” in a recipe but it didn’t feel right and I looked it up. Much to my astonishment, it seems it’s just a single word – lukewarm.
I’ve also no idea what temperature constitutes lukewarm. In my mind, it is definitely warmer than room temperature but colder than anything that could be considered warm. Perhaps it is a state of mind.