hlēow + wearm

Old English had a word, hlēow, that meant warm, tepid, or sheltered from the wind. It described the kind of warmth that was comfortable rather than hot — a sheltered spot, a mild day, body-temperature water. Over time, hlēow became leuk or luke in Middle English, and its meaning narrowed to specifically tepid, neither hot nor cold.

At some point — probably around the 1300s — English speakers stopped recognizing luke as a standalone word and bolted warm onto it for clarity. Lukewarm was born. It was a tautology: warm-warm. But nobody noticed, because luke had faded from common speech. The redundancy felt like a compound, not a repetition, and so it stuck.

This kind of accident happens in English regularly. The River Avon is 'River River,' because avon is the Celtic word for river. Torpenhow Hill in Cumbria may be 'Hill Hill Hill Hill' in four languages stacked on top of each other. Lukewarm is the domestic version of the same pattern — a forgotten word propped up by its own synonym.

The word has never been exciting, which is appropriate for something that describes the absence of extremes. Lukewarm water, lukewarm enthusiasm, lukewarm reviews — the word always implies insufficiency. Not cold enough to be refreshing, not hot enough to be useful. English accidentally created a word that sounds exactly as uninspiring as the temperature it describes.