Picture one bag of sand on the ground. It is heavy for its size, it stays exactly where you drop it, and it settles into whatever is under it and fills the gap. Pick it up and you feel the weight before you are ready for it. Those two things — it blocks, and it is heavier than it looks — are what the word keeps carrying when it leaves the flood and moves into meetings and games.
Simply sand plus bag. The compound says exactly what the thing is, and every later use still leans on that object.
就是 sand（沙）加 bag（袋子）拼起來，字面跟實物完全一樣。值得注意的是後面那些抽象用法都還靠著這個實物在運作：擋住、壓著、突然變重。
Wiktionary entry for sandbag, packet source_ref kaikki:3f6f69a190036ae4:1174054789 — 「From sand + bag.」
Three branches
實物與防護：把水和衝擊擋在外面
這一族就是核心畫面本身，先是那袋沙，然後是拿它去擋的動作。
noun · B2
A strong bag filled with sand. People stack them in rows to hold back floodwater, or pile them into a thick wall that protects a building or a person from an explosion and from flying pieces.
沙包
比喻：突然被壓住，來不及反應
從那袋沙的重量和突然感借過來：東西壓下來的時候，人根本來不及擋。
verb · C1
To treat someone unfairly so that they cannot do what they wanted or cannot succeed — usually by hitting them with something they had no chance to prepare for, or by working against them quietly.
暗算
比喻：把自己的實力壓在底下
同樣是「壓著」，但壓的是自己的力氣；來源的資料把這條線追到牌桌上。
verb · C1
To hide how good or how strong you really are by doing less than you can, so that other people expect little from you and you gain an advantage later. It is also used about numbers: to set a target lower than you know you can reach.
藏實力