It turns up in the 15th century as a collective noun; Rosemary Horrox suggests it was "coined," meaning that it was deliberately made up rather than naturally evolved. Starkey cites it in his book The English Court. If you look at the Old English etymology, "threat" once meant "crowd." I don't see any references to actual Tudor people calling any extant group of courtiers or court faction by this term, though.
Just endorsing Foose's coment about etymology - it turns up eg in the Old English poem 'Wulf and Eadwacer'. I suspect it has overtones of 'troop' or 'gang', though, not just 'crowd'.
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