Un ange passe....
At first sight, all words in this expression are easy to translate from French into English: un ange passe - an angel is passing. Easy, peasy. The question is: when exactly, in every day life, would you feel a need to point out that an angel that may have been hanging out in the neighborhood, decided to drop by, and then move on?
This is what you would call a context sensitive expression, i.e. an expression that only makes sense if used in the right context. The French use un ange passe to describe a sudden, unexpected break in an otherwise lively conversation in a group of 3 or more people. The sudden silence may be due to some kind of embarrassment caused by the conversation itself, or by an external factor such as someone entering the room. Or it just happens that several streams of conversation end at the same time, and what actually causes the awkwardness is the silence that ensues. The fact is, saying un ange passe is a way to avoid a prolonged silence by breaking it. It immediately lightens up the atmosphere and brings about smiles on people's faces.
Origin
There is no certainty as to the exact origin of the expression but it seems to have been in use since ancient times. The main theory is that it comes from an ancient Greek locution involving Hermes, the herald of gods. In Greek mythology, Hermes was considered the emissary and messenger of all other gods, and it was forbidden to talk in his presence - hence the reference to silence. As Christianity disseminated throughout Europe, Hermes was gradually replaced with the proverbial angel, who was clearly more compatible with the Christian religion.
One may wonder: is the angel the cause or the effect of the silence? On the one hand, the fact that an angel is passing might be in itself distracting enough for the conversation to dry up. On the other hand, it could mean that the angel is aware of the awkwardness and is passing in order to try and smooth it over. Isn't it what angels do, after all?
Examples
La conversation était animée, quand soudain...tout le monde s'est tu. Un ange est passé.
The conversation was lively, when suddenly...everyone fell silent. It felt awkward.
En annonçant que j'avais perdu mon emploi, un ange est passé.
When I announced that I'd lost my job, there was an awkward silence.
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