"In no uncertain terms" is an idiom in which the "no" and the "un-" cancel, so that the result means something like "in very specific and direct language", "very clearly", "in a strong and direct way", or perhaps "emphatically". In other words, "in no uncertain terms" means "in certain terms", construing "certain" as in certainty. The earliest example that I've been able to find is this sentence from the Chicago Tribune, July 20 1863:
Our dispatches contain another circular from the Provost Marshal General's office, and accompanying, the voice of the Government, couched in no uncertain terms, that the draft will be enforced in every loyal State, without fear or favor.
And "in no uncertain terms" is still being used that way, as in this example from today's New York Times:
After last week, the question now is: Why am I writing a post this week instead of sleeping?
When more than 200 people tell you, in no uncertain terms, that the first step to dealing with the exhaustion incurred when a child does not sleep is to find ways and moments for you, yourself, to sleep, that’s a fair question.
But recently, through the miracle of misnegation, this elderly cliché has found a new role in life.
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