The word 'tepidly' originates from the Latin adjective tepidus, meaning lukewarm, which itself derives from the verb tepēre, to be moderately warm. This Latin root is traced further back to the Proto-Indo-European base tep-, meaning to be warm, which also gives rise to words like 'tepidarium' (a warm room in Roman baths). The adverbial suffix '-ly' is of Germanic origin, entering English from Old English. Thus, the literal construction "in a lukewarm manner" evolved naturally to describe not only physical temperature but also metaphorical lack of warmth or enthusiasm in action.

Structure: tepid (lukewarm) + ly (adverb suffix) - tepid: From Latin tepidus (lukewarm, tepid), from tepēre (to be warm). It functions as the adjectival root conveying the core meaning of moderate warmth. - ly: From Old English -līce (forming adverbs from adjectives). It functions as a derivational suffix that converts the adjective into an adverb.