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.
From the word’s history
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, "transom"
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