Something big stands right next to something smaller and rises far above it, so its shadow falls across the smaller thing. The small thing is still there, still whatever it was. It is just standing in the shade now, and the light is not reaching it.
Wiktionary traces it to Old English ofersceadwian, literally over plus shadow: to cast a shadow over something. The modern uses all still sit on that literal picture.
Wiktionary 追到古英文 ofersċeadwian，就是 over（在上面）加 shadow（影子），字面是「把影子投在上面」。今天不管是山壓著村子、名氣壓著弟弟，還是壞消息壓著一整趟旅行，用的都還是這個字面畫面。
Wiktionary entry supplied in evidence packet (source_ref kaikki:3f6f69a190036ae4:1057924927:26823): from Middle English overschadwen, from Old English ofersceadwian, over- + shadow.
Two branches
看得見的影子：高高壓在上面
這一族就是畫面本身：東西是真的又高又近，影子也是真的。
verb · C1
If a mountain, a tall building, a big tree, or anything else very large overshadows a place, it stands close to it and rises far above it, so that it hangs over the place and keeps the light off it.
高聳遮蔽、罩在上方
影子落在人和事情上
把實體的影子換成看不見的影子：一邊太大、太亮或太沉重，另一邊就進了陰影——不是變差，是照不到光。
verb · B2
To take so much of the attention that a person or thing beside you stops being noticed and starts to seem small or unimportant, even when it is good in itself.
蓋過、使相形失色