One way to hold sullen: someone has shut themselves off. The face is set, the mouth is closed, and the back is half turned to the room. Nothing is being said, yet everyone can feel that something is wrong and that it will not be explained. The word began as a word for being alone, and that shutting-off is still the picture inside it. Move the same picture onto the weather and you get the second use: a sky or a sea that has gone dark, low and silent, holding something back that has not arrived yet.
From the word’s history
From Middle English soleyn, from Anglo-Norman soleyn 'alone', from Old French sole and Latin sōlus 'by oneself alone'. The meaning moved from 'single, alone' to 'morose' during the Middle English period.
來自中古英文 soleyn，再往上是盎格魯-諾曼語 soleyn（獨自的）、古法文 sole 和拉丁文 sōlus（一個人）。從「單獨」轉成「陰鬱、鬧脾氣」，是在中古英文時期發生的，所以「把自己關起來」這個畫面不是後來附會的比喻，而是這個字走過的路。
Wiktionary entry supplied in this packet (source_ref kaikki:3f6f69a190036ae4:1093405175)
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