手臂底下那一塊身體
這一組就是核心畫面裡「位置」的那一半：手臂和身體相接的凹窩，以及所有貼著那個位置說的事——毛髮、汗、衣服的接縫、專門做給那裡用的產品。
noun · B1
The hollow place under your arm, where your arm joins your body.
腋下；腋窩
核心畫面
The word is just under plus arm, and you can think of two pictures sharing that one phrase. One is a place: the soft hollow between your arm and your side, the spot a goes into and a is made for. The other is a path: a hand that stays below the shoulder, swinging forward from underneath and letting the ball go while it is still low. The first is about where something sits; the second is about how high a hand travels. They are held together by the words under the arm, and that is a way of remembering them, not a claim that one grew out of the other.
這個字就是 under（在…底下）加上 arm（手臂）。可以想成有兩個畫面共用這一句話。一個是位置：手臂和身體之間那…更多收起
這個字就是 under（在…底下）加上 arm（手臂）。可以想成有兩個畫面共用這一句話。一個是位置：手臂和身體之間那個柔軟的凹窩，量體溫夾的地方，體香劑針對的地方。一個是路線：一隻手一直待在肩膀以下，從底下往前送，放開球的時候還是低的。前者講東西在哪裡，後者講手抬到多高。把它們放在一起看，是因為都貼著「手臂底下」這句話；這是記憶的方式，不等於說其中一個是從另一個變來的。From the word’s history
Wiktionary records underarm simply as under- plus arm. It adds that the armpit meaning is not old: it appears in the 1920s, and is believed to have arisen as a euphemism in advertising. That fits where the body-part senses still live today, on labels and in health notices rather than in ordinary speech.
Wiktionary 記載 underarm 就是 under- 加 arm。它還提到，指腋下的那個意思其實不老：大約 1920 年代才出現，一般認為是廣告裡的委婉說法。這剛好對得上今天的情況——講身體的那幾個用法主要活在包裝、標示和衛教文字裡，日常講話的人還是說 armpit。
Wiktionary via kaikki, source_ref kaikki:3f6f69a190036ae4:1144656625: From under- + arm. The armpit sense is not old, historically speaking (1920s), and it is believed to have arisen as a euphemism in advertising.
Two branches
手不過肩的那一送
這一組和上面那組共用 under the arm 這句話，但「底下」指的不是同一件事：上面講的是身體上的位置，這裡講的是手走過的高度。兩者放在一起是方便記，不是說誰從誰變來。LDOCE 把其中的副詞標為英式英語。
adjective · B2
Describing a throw, a serve, or a bowl in which the hand stays low: the arm swings forward from below and never goes above the shoulder.
低手的；下手的