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.