One way to hold this word: merrily is brightness you can see or hear in something that is happening. Not a feeling kept inside, but a laugh, a song, a wave, a , a tick. Because it is an adverb, it always hangs on an action, never on a person. Once you have that picture, the other uses are easy to follow. Lend the brightness to a fire or a stream and you get the second use. Lend it to someone who should be worried and is not, and the same word turns into a quiet complaint.
From the word’s history
The packet's Wiktionary entry gives merry + -ly, from Middle English myriely and Old English myriġlīċe. The -ly ending is what makes it an adverb, and that is why the word always describes how something is done rather than what someone is like.
來源把它記成 merry 加 -ly，再往前接到中古英語的 myriely 和古英語的 myriġlīċe。-ly 這個字尾把形容詞變成副詞，這正好解釋了為什麼 merrily 一直在講「怎麼做」，而不是「這個人是什麼樣子」。
Wiktionary entry for merrily, supplied in this packet (source_ref kaikki:3f6f69a190036ae4:1182317453:8049): From Middle English myriely, from Old English myriġlīċe; equivalent to merry + -ly.
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