ふと、なんで具合悪い時にI'm feeling unwellの代わりに大抵の人はunder the weatherっていうのかな？とオリジンを探してなるほどと思った。
The phrase “under the weather” originated in the early 1800s and was initially used by sailors. When a sailor was feeling seasick or unwell, they would often go below deck to escape the elements and recover, quite literally putting themselves “under the weather” on the deck above.

はい、今日はI'm a bit under the weather.　二日酔いでも船酔いしているわけではありません😅