A cup of tea that was hot an hour ago. You pick it up and your hand feels almost nothing — not heat, not cold, just a little warmth left over. That leftover warmth is the whole word, and it is what makes tepid sound slightly disappointing wherever it turns up.
The source derives the word from Latin tepidus, which already meant slightly warm, so the temperature sense is the older, literal centre and the feelings sense reads as that same temperature moved onto people.
來源資料說這個字借自拉丁文 tepidus，本來就是「微溫」。所以溫度那一邊是字面上的核心，講反應的那一邊可以看成同一個溫度被搬到人身上。
Wiktionary etymology supplied in this packet: "Borrowed from Latin tepidus." (source_ref kaikki:3f6f69a190036ae4:316852386)
From the picture
感覺：不冷不熱的反應
同一個溫度，換到人身上。熱情就像熱水一樣會涼掉，所以英文用 tepid 來說一群人的反應只剩一點點溫度：掌聲響兩下就停，評論不好不壞。
adjective · C1
Showing very little excitement, interest or support. A tepid response, welcome or review is polite but flat — people did not hate the thing, they simply were not moved by it.
冷淡的；不熱烈的
adjective · C1
Only slightly warm — not hot and not cool. It is used about drinks, water and food, often when the thing should have been hot and has lost almost all of its heat, which makes it unpleasant.
微溫的