I'm looking for an idiom that describes a completely veiled threat, like when someone says something that seems perfectly polite, but a person who is familiar with the situation would know that what they are saying actually has a sinister double meaning.
The closest thing I've come up with so far is, "iron fist in velvet glove", but that isn't quite what I want. Cambridge Dictionary says
used to describe someone who seems to be gentle but is in fact forceful and determined
I feel like I've heard an idiom that is something like "a velvet shrouded sword", and that it is a literary reference, but I can't find anything about it online, so perhaps I imagined it.
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I am asking for any synonyms of "Iron fist in velvet glove" and to know whether the expression, "velvet shrouded sword" sounds familiar to anyone