借花苞的形狀和顏色形容嘴
只借畫面的外形：小、圓、紅。
noun · C1
Used in 'rosebud mouth' and 'rosebud lips' to describe a mouth that is small, round and very red, like a rose that has not opened.
櫻桃小嘴（小而紅潤的嘴唇）
核心畫面
A picture to hold: a rose that has not opened. Small, tightly rounded, a deep concentrated red, and clearly not finished yet. Everything else the word does can be seen as someone holding that bud up next to something else, a small red mouth, or a young person.
可以想成一顆還沒打開的玫瑰：小小的、圓圓的、緊緊捲著，紅得很集中，而且看得出來「還沒開」。這個字的其他用法，大多可以…更多收起
可以想成一顆還沒打開的玫瑰：小小的、圓圓的、緊緊捲著，紅得很集中，而且看得出來「還沒開」。這個字的其他用法，大多可以想成有人把這顆花苞舉起來，拿去比別的東西，比一張小而紅的嘴，或比一個年輕的人。From the word’s history
The source gives the formation directly: rose plus bud. Both parts keep their plain meanings, which is why the picture stays so clear.
來源直接說明了組成：rose 加 bud。兩半都保留原本的意思，所以畫面才這麼清楚。
Wiktionary etymology supplied in this packet (kaikki:3f6f69a190036ae4:781389817:19753)
From the picture
借花苞的「還沒開」形容人
借的不是外形，而是階段：還年輕、還沒展開。
noun · C1
A pretty young woman, spoken of as a rose that has not opened yet; the source marks this as British and says it is sometimes used as a fond name for someone.
年輕漂亮的女孩
Also in this word
noun · B1
A rose flower that has not opened yet: a small, tight, round head on the stem, with the petals still folded inside.
玫瑰花苞