A lid pressed down until there is no gap anywhere round the edge. Nothing can slip in, and nothing can slip out. Everything this word does grows from that one seal: first a real jar keeping the air off the tea, then a plan or a story shut so tightly that no doubt can get a finger into it.
可以想成一個蓋子壓下去，沿著邊緣一圈完全沒有縫。外面的東西進不來，裡面的東西也跑不掉。這個字的兩個用法都從那道封口長出來：先是真的罐子，把空氣擋在茶葉外面；接著是一個計畫、一段說詞，關得太密，連一絲懷疑都插不進去。記住「縫」這個字，兩邊就串起來了。
From the word’s history
From air + tight — literally 'tight against air'.
維基詞典記載它由 air（空氣）加 tight（緊、不留縫）組成，字面就是「對空氣來說很緊」，正好對上那道沒有縫的封口。
Wiktionary entry for airtight (packet etymology_text: From air + tight.)
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