Think of one lying on a table: long, thin, narrow, and worth almost nothing. Every other use of shoestring seems to borrow one feature of that little cord. Some borrow its shape — a strip of land, a thin-cut fry. One borrows its position, down at shoe level, for a catch made just above the grass. And the best-known ones borrow its cheapness: money so thin there is nothing spare.
可以想成一條攤在桌上的鞋帶：又細、又長、又不值錢。shoestring 的每個用法，都像是從這條繩子身上借走了一個特點。有的借形狀，像細長的一塊地、切得很細的薯條。有的借位置，球低到鞋帶那裡，就成了貼地的接殺。最有名的那組借的是它的便宜——錢少到像一條繩子，剛好夠用，一點都不剩。把這條繩子記在腦子裡，這幾個看起來不相干的意思就連起來了。
From the word’s history
The noun is formed from shoe + string, so the word literally names the string of a shoe. That plain compound is also the picture the figurative uses run on.
這個名詞由 shoe（鞋）加 string（細繩）組成，字面就是「鞋子的繩」。來源明確說明了這個組成方式。比喻用法用的也正是這個字面畫面。
Wiktionary, "shoestring"
登入就可以看每一支怎麼連回核心畫面。