Most of us know that lukewarm means tepid, moderately warm. But the term is another of those strange ones to the modern ear. What the heck does luke have to do with temperature?
This is another case where an archaic word remains fossilized in a word still in use. Luke is a Middle English form of the Old English hléow meaning warm or sunny. From a poem c.1205:
And opened wes his breoste. Tha blod com forth luke.
(And his breast was opened. The bood came forth luke.)
By the late 14th century, the form lukewarm had made its appearance, with a shift in meaning to tepid. From John de Trevisa’s Bartholomeus of that year:
The broth of clete...comfortyth the teeth: yf it be luke warme hote holde in the mouth. [One manuscript reads "lewke hote."]
(The broth of burdock…comforts the teeth: if it is held in the mouth lukewarm.)
(Source: Oxford English Dictionary, 2nd Edition)
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