Lukewarm; only slightly warm, or lacking enthusiasm and interest.
From Latin 'tepidus' (warm, lukewarm). The word has meant 'moderately warm' since its entry into English around the 1500s, and metaphorically 'lacking intensity' since the 1600s.
Humans have a remarkable sense for 'tepid'—we can detect temperature changes of just 0.01 degree Celsius on our skin, which is why tepid water feels distinctly disappointingly 'meh' compared to hot or cold.
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