I find it means something like "sudden rage", but i am a bit confused about the usage:
Syrie : le coup de sang de l'ambassadeur de France
in this article on lefigaro.fr. The ambassador doesn't seem really mad in the video...
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Un ambassadeur maitrise ses émotions, ça fait partie de son personnage. Il ne faut pas juger de son coup de sang à ce qu'on voit mais à ce qui est dit. Il prononce scandale et scandaleux trois fois en trente secondes : ça suffit à mon avis pour dire qu'il a un coup de sang !
The ambassador might not look as if in a rage because mastering one's own feelings is part of an ambassador's training. So you mustn't judge on looks but on what is being said. If an ambassador says scandale (scandal) and scandaleux (scandalous) three times in thirty seconds you can say he is outraged.
"Coup de sang" is used in various vague definitions because it is in fact a vague and sensationalist descriptive term which has all the hallmarks of a hype word. "Coup" is used for guns, punches, drinks, strikes, thunder, kicks, elbowing someone, damage. It is a word which is synonymous with drama and sudden alarm, a disorienting word for a user to filter cognitively it's a shock word. it is not precise or academic.
The figure of speech can be compared to "coup de coeur", which is used a lot in advertising and sales, to justify why someone would want to buy something.
Recently I saw "coup de sang" used in the news regarding a man on holiday who had a phase of dizziness and next day stabbed three girls and their mum with a Swiss-army-type knife, puncturing a little girl's lung, on a holiday resort with his wife and two kids... Alledgedy because of a brain malfunction to do with aneurism...
I haven't heard the term in 30 years of French fluency; it's an example of the functional quirkiness of modern French in a flashy media context and in a search for new form and function.
AVC is a unappealing term that makes people uncomfortable, because French needs a lot of momentum and song within it's phrases to ring true; people refer to it instead as "attaque cerebrale" and other variations, perhaps part of the reason for the "coup de sang" existing. In English, it makes sense to discuss the ins and outs of strokes informally, but you won't have long conversations about AVC's, the term in itself inhibits good information exchange.