Lukewarm

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late Middle English: from dialect luke (probably from dialect lew ‘lukewarm’ and related to lee) + warm.

wiktionary

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From Middle English leukwarm, lukewarm(“lukewarm, tepid”), equivalent to luke(“lukewarm”) +‎ warm. Compare Saterland Frisian luukwoarm(“lukewarm”), German Low German luukwarm(“lukewarm”), German lauwarm(“lukewarm”). First element believed to be an alteration of Middle English lew(“tepid”) (> English dialectal lew), from Old English hlēow(“warm, sunny”), from Proto-Germanic *hliwjaz, *hlēwaz, *hlūmaz, *hleumaz(“warm”), from Proto-Indo-European *ḱal(w)e-, *ḱel(w)e-, *k(')lēw-(“warm, hot”). Cognate with Dutch lauw(“tepid”), German lau(“lukewarm”), Faroese lýggjur(“warm”), Swedish ljum(“lukewarm”), ljummen(“lukewarm”) and ly(“warm”), Danish lummer(“muggy”), Danish and Norwegian lunken(“tepid”), dialectal Swedish ljummen(“lukewarm”).

etymonline

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lukewarm (adj.)

"neither cold nor hot, tepid," late 14c., from warm (adj.) + luke (adj.) "tepid" (c. 1200), a word of unknown origin.

Figurative sense of "lacking in zeal, not ardent" (of persons or their actions) is from 1520s. Related: Lukewarmly; lukewarmness. Luke-warmth (1590s) is marked "rare" in OED.