What's the term when you say something like "90% chance this is not true". But you actually don't know the percentage?
Is it "figure of speech"?
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The most accurate term for such arbitrarily assigned percentages may be "false precision"— which consists of indicating a degree of exactitude that significantly exceeds the objective basis for assigning a probability or size estimate to a potential outcome or quantity.
As such, false precision seems to be a species of what Wilson Follett, Modern American Usage (1966) refers to as scientism:
scientism 1. The deserved admiration in which science and technology are held today has had the effect of making the lay world want to share in their reputation by borrowing their trappings. The result is all around us in many forms, from the design of household objects to the new hybridized vocabularies of trade and the professions. To be trusted, everything must wear the technical, the scientific, look. The frame of mind and the feeling that produce and reproduce these imitations, crude or subtle, are properly scientism.
Clearly, the motive for saying, off the top of one's head, that "there is a 90% chance that [some assertion] is not true" rather than that "it is highly likely that [some assertion] is untrue" is the implied rigor and confidence that comes with pegging a probability within two decimal places of numerical specificity. The assertion sounds authoritative and suggests that the estimate rests on some sort of computation, rather than on a generalized sense of likelihood based on personal experience or intuition. In a word, it sounds scientific, even though there may be no serious science underlying it.
Used informally, it is a figure of speech, as defined by Richard Nordquist at Thoughtco.com:
In common usage, a figure of speech is a word or phrase that means something more or something other than it seems to say—the opposite of a literal expression....
For the purposes of this answer, the phrase the opposite of a literal expression is key. If you say, informally, that the probability of X (or the probability of not-X) is 90%, all you are saying is that X is extremely likely, (or unlikely). No one expects you to support your statement by a statistical analysis of data that you have examined and can vouch for. This usage of "90 percent likely" is absolutely not a lie!
However, if you are giving a scientific estimate, such a statement could be sloppy, could result from misunderstanding the subject (e.g., a paper that you have quickly scanned), could be a shading of the truth, or could be an outright lie. And, it might be a truthful summary of the best estimate you can give, based on the data, in which case it would be followed by a discussion of the limitations of the data and the need for further research.
The OP is not clear on which context -- the informal or the scientific -- he is referring to, but it is (90% ?!) likely that he means the informal context.