真的關到不透氣
這一支就是核心畫面本身，講的是看得到、摸得到的封口。
adjective · B2
Closed so tightly that no air can get in and no air can get out. It is used about containers, lids, seals, rooms and bags — anything shut well enough to keep gas from passing through.
密封的；氣密的
核心畫面
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…更多收起
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.)
Two branches
找不到破綻
把同一道封口搬到摸不到的東西上：論點、計畫、證據也可以「沒有縫」。維基詞典把這個用法標為比喻用法（figuratively）。
adjective · B2
Planned, written or put together so carefully that there is no weak spot — no gap where a problem, a doubt or an objection could get in. It is used about arguments, plans, cases, alibis, excuses and rules.
滴水不漏的；無懈可擊的