An American acquaintance who had used the phrase just in case
twice in a technical article about the philosophy of linguistics was asked
twice by the copy editor what he meant by it. A reasonable question
if (as I suspect) the copy editor was British. There is an unusually
nasty dialect difference in connection with this phrase.
一位美国熟人曾在一篇关于语言学哲学的技术文章中两次使用了“just in case”这个短语，结果被编辑两次询问他具体是什么意思。如果（正如我猜测的那样）这位编辑是英国人，那么这个问题是合理的。围绕这个短语存在一个异常棘手的方言差异。
In British English (and, let me say by way
of update, also for most ordinary Americans),
just in case always means "lest".
Thus We'll take an umbrella just in case it rains means nothing
other than "We'll take an umbrella in order to insure ourselves
against the unfortunate possibility that it might rain."
In the Everly Brothers'
song "Just In Case", on their superb 1960 album
It's Everly
Time, the title clearly has this sense.
在英式英语中（顺便说一下，作为更新，这种用法也适用于大多数普通的美国人），“just in case”总是意味着“以防万一”。因此，“We'll take an umbrella just in case it rains”（我们会带上一把伞以防下雨）只意味着“我们会带上一把伞，以防备可能下雨的这种不幸情况”。在埃弗里兄弟（The Everly Brothers）1960 年那张出色的专辑《It's Everly Time》中的歌曲《Just In Case》中，标题显然具有这种含义。
But in American English as used by those trained in the formal sciences
and philosophy (and perhaps also now
by some British writers who have picked up the
usage of American scholars), just in case
has acquired a new idiomatic sense:
"in, and only in, the case where", or in other words, "if and only if".
So a sentence like An integer greater than 1 is a prime
just in case it has no divisors other than 1 and itself
means "An integer greater than 1 is a prime
if and only if it has no divisors other than 1 and itself."
但在美国英语中，对于那些受过形式科学和哲学训练的人（也许现在一些受到美国学者影响的英国作家也开始使用这种表达方式），“just in case”获得了一种新的习惯用法意义：“在且仅在某种情况下”，换句话说，就是“if and only if”（当且仅当）。因此，像这样的句子：“An integer greater than 1 is a prime just in case it has no divisors other than 1 and itself”（大于 1 的整数是素数，当且仅当它除了 1 和它本身之外没有其他因数）意味着：“An integer greater than 1 is a prime if and only if it has no divisors other than 1 and itself.”（大于 1 的整数是素数，当且仅当它除了 1 和它本身之外没有其他因数）。
I have always thought this is a disastrous piece of dialect divergence.
Imagine an American computation theorist talking to a British English
speaker who has lived a sheltered life:
我一直认为这是一个灾难性的方言分歧。想象一下，一个美国计算理论学家与一个过着与世隔绝生活的英国英语使用者交谈的情景：
A: A set of of strings has a decidable membership problem just in case it and its complement are both recursively enumerable.
一组字符串的成员资格问题是可判定的，当且仅当该集合及其补集都是递归可枚举的。B: What, you mean the set acquires decidability to guard against the worrying eventuality that it and its complement might turn out to be be r.e.? You must be bonkers.
B：什么，你是说这个集合获得了可判定性，以防它和它的补集可能会变成递归可枚举的？你一定是疯了。
Or imagine a British English speaker planning a picnic with an
American logician whose horizons are entirely limited to within logic
and philosophy (someone totally unacquainted with the Everly
Brothers or other ordinary people):
或者想象一位讲英式英语的人正在与一位美国逻辑学家计划一次野餐，而这位逻辑学家的视野完全局限于逻辑和哲学领域（完全不了解埃弗里兄弟或其他普通人）：
B: We'll take an umbrella just in case it rains.
B：我们带把伞以防下雨。A: What? Material equivalence holds between us taking an umbrella and it raining? What're you talking about? I don't see an implication in either direction, given that the second proposition is a contingent claim about the future; let alone a biconditional... You gotta be kidding.
A: 什么？我们带伞和下雨之间存在材料等价？你在说什么？我看不出任何方向上的蕴涵关系，尤其是第二个命题是关于未来的或然性主张，更别提一个双条件了……你一定是在开玩笑吧。
I do not normally have much regard for the old saying about the British
and the Americans being two nations divided by a common language; but it
really does come to mind in this restricted case.
我通常并不太在意关于英国人和美国人是被共同语言分隔的两个国家的那句老话；但在这个特定情况下，它确实浮现在我的脑海中。
[Please note the green updates above.
I am not saying that no American sources have the "lest" sense.
I am saying that American logicians, mathematicians, philosophers,
and such have departed from it. Try a Google search on this pattern:
[请注意上方的绿色更新。我并不是说没有美国来源具有“lest”的含义。我是说美国的逻辑学家、数学家、哲学家等已经不再使用这种含义。试着用以下模式进行谷歌搜索：
"just in case" site:plato.stanford.edu
That will bring up a slew of examples of the "if and only if"
meaning.]
这将会带来大量关于“当且仅当”含义的例子。