The expression is delightfully ambiguous.
The word for message
can mean word
as well, but can also mean thing,
object,
even experience.
We as readers of course know that Ehud is referring here to his sword hidden on his right thigh, but stupid Eglon naively interprets the word in its most common sense. Perhaps he expects an announcement of additional gifts from Israel or a report of rebellion in his subject territories.
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But Ehud did have a secret message from God for the king. That message is 18 inches or 45 centimetres long, sharp as a two-edged sword. And Eglon will get the point of the message. For Ehud’s words, like his dagger, are double-edged.
19 But he himself turned back at the idols near Gilgal and said, “I have a secret message for you, O king.” And he commanded, “Silence.” And all his attendants went out from his presence.