adjective
describing something that is unbelievable or difficult to accept
preposition
connecting the words 'not' and 'to be believed', indicating the relationship between them
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The information provided in the report is not to be believed until further verification is conducted.
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Don't trust everything you hear about that gossip, it's not to be believed.
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That story is so wild, it's not to be believed!
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His excuses for being late were so ridiculous, they were not to be believed.