能力的外圍界限
把「包住一圈」的那條線理解成「能力範圍的邊界」：邊界之內是做得到的，把它往外推，就是去試以前做不到的事。
push the envelope · noun · B2
in the phrase 'push the envelope': to try to go past the usual limits of what people do or what is possible.
突破極限（只用於 push the envelope）
核心畫面
A thin skin wrapped right round something, closing it in on every side — the paper round a letter, a layer of gas round a planet. Once you see it as 'the boundary that encloses', the idiom about limits stops looking random: in aviation the outer edge of what an aircraft can safely do was called its flight envelope, and pushing that edge outward is what the phrase describes.
可以想成：一層薄薄的皮，四面八方把東西包起來、封在裡面——信外面那張紙、行星外面那層氣體。一旦看成「把東西圍住的那圈…更多收起
可以想成：一層薄薄的皮，四面八方把東西包起來、封在裡面——信外面那張紙、行星外面那層氣體。一旦看成「把東西圍住的那圈界線」，那個講極限的片語就不再突兀了：航空上把一架飛機能安全做到的範圍叫作 flight envelope（飛行包絡線），把那圈邊界往外推，就是 push the envelope 這句話的畫面。From the word’s history
Merriam-Webster traces 'push the envelope' to aeronautics, where the flight envelope is an aircraft's outer limits of safe performance; the phrase appeared in print in Aviation Week in 1978 and was popularised by Tom Wolfe's The Right Stuff (1979).
根據 Merriam-Webster，push the envelope 來自航空領域：flight envelope（飛行包絡線）指一架飛機能安全運作的外圍界限。這個說法 1978 年見於 Aviation Week，並因 Tom Wolfe 1979 年的 The Right Stuff 而流行起來。所以這裡的 envelope 是「界線」，不是信封。
Merriam-Webster, Origin of 'Pushing the Envelope', https://www.merriam-webster.com/wordplay/push-the-envelope-idiom-space-aeronautics-origin
From the picture
Also in this word
letter cover · noun · A2
a thin paper cover that you put a letter or card inside, so you can send it to someone.
信封
surrounding layer · noun · B1
a layer of something that covers or surrounds another thing on every side.
包覆層；外層
原始詞典資料13 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
envelopenoun詞源 1/ˈɛnvə(ˌ)ləʊp/
- 1
A paper or cardboard wrapper used to enclose small, flat items, especially letters, for mailing.
- Now we are liberal with our innermost secrets, spraying them into the public ether with a generosity our forebears could not have imagined. Where we once sent love letters in a sealed envelope, or stuck photographs of our children in a family album, now such private material is despatched to servers and clouds operated by people we don't know and will never meet.
- 2
Something that envelops; a wrapping.
- 3
A bag containing the lifting gas of a balloon or airship; fabric that encloses the gas-bags of an airship.
- They have no internal or external support structure, being simply a fabric bag (or envelope) filled with a lighter than air gas. Inside the envelope are one or more "ballonets", or smaller bags, which help maintain the envelope's shape.
- 4
geometrymathematicssciencesA mathematical curve, surface, or higher-dimensional object that is the tangent to a given family of lines, curves, surfaces, or higher-dimensional objects.
- 5
businesselectrical-engineeringelectricityA curve that bounds another curve or set of curves, as the modulation envelope of an amplitude-modulated carrier wave in electronics.
- 6
entertainmentlifestylemusicThe shape of a sound, which may be controlled by a synthesizer or sampler.
- 7
computingengineeringmathematicsThe information used for routing a message that is transmitted with the message but not part of its contents.
- 8
biologynatural-sciencesAn enclosing structure or cover, such as a membrane; a space between two membranes
另外 3 個記錄的用法
- 9
engineeringnatural-sciencesphysical-sciencesThe set of limitations within which a technological system can perform safely and effectively.
- push the envelope
- Few of the lads had ever been in combat and they knew little about the critical tolerances of fighter aircraft during violent maneuvers. They knew where the outside of the envelope was, but they didn't know about the part where you reached the outside and then stretched her a little . . .
- 10
astronomynatural-sciencesThe nebulous covering of the head or nucleus of a comet; a coma.
- 11
An earthwork in the form of a single parapet or a small rampart, sometimes raised in the ditch and sometimes beyond it.
- make a blind all along the bottom of the ditch of the Envelope
envelopeverb詞源 1/ˈɛnvə(ˌ)ləʊp/
- 1
rareTo put (something) in an envelope.
- Arthur Armytage drew the precious document from his bureau; and without trusting himself to a re-perusal, enveloped and re-enveloped—sealed and resealed it;—mounted his horse, and rode off to Greta Castle.
- This business of returning letters to the writers gives occupation to about sixty lady clerks in the “Return-Letter Department,” in the gallery up-stairs. To them belong the duties of reënveloping the letters in the well-known, yellow, dead-letter envelope, by which, under an act of Congress approved June 12, 1866, these are returned to the writers free of charge.
這個拼法也被記錄為下列字的變化形：
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。