c.1400, from Latin tepidus "lukewarm," from tepere "be moderately warm," from PIE root *tep- "to be hot" (cognates: Sanskrit tapati "makes warm, heats, burns," tapas "heat, austerity;" Avestan tafnush "fever;" Old Church Slavonic topiti "to warm," teplu "warm;" Old Irish tene "fire;" Welsh tes "heat"). Related: Tepidly; tepidity.