You can think of transom as one piece laid straight across an opening. A bar going side to side over a door. A beam going side to side across the back of a boat. Whatever it crosses, it holds the two sides apart and steady, and it marks a line: below it one thing, above it another. Once that line exists, people start naming what sits on it — the little window above the door, the flat panel at the back of the hull — with the same word.
Middle English traunsom, probably an altered form of Latin transtrum, a crossbeam. The Latin word was used for the crossways timbers of a boat, which is exactly where the modern boat senses still sit.
來源是中古英文 traunsom，很可能是拉丁文 trānstrum（橫樑）變來的。拉丁文那個字本來就用在船上橫著的木頭，所以今天船的用法等於一直待在原地。建築上的橫楣是同一個畫面換了地方：同樣是一根橫著架過去的料。
Wiktionary entry for transom (noun), packet source_ref kaikki:3f6f69a190036ae4:902783228:14840 — 「From Middle English traunsom; probably an alteration of Latin trānstrum (crossbeam).」
Two branches
建築：門窗上的那根橫料，和它上面的小窗
這一族最貼近核心畫面：一根橫著架過去的料，架在門窗的開口上。
noun · C2
A bar of wood, stone or metal that runs straight across a window or a door frame. It either separates a door from the window fitted above it, or divides a single window into an upper part and a lower part.
橫楣／橫櫺
船：船尾的橫樑，和整片平平的船尾
還是同一根橫著架過去的料，只是地點從門窗換到船的後端；拉丁文原本講的就是船上的橫木。
noun · C2
A beam of wood that runs across the back of a boat and forms the top edge of the stern.
船尾橫樑