How do you say "not a credible person"?
How do you say "She said she would give me the comments by the the end of three weeks ago but she didn't. I sent her a kind reminder in the next three weeks, she said she would, but still didn't. She is not a credible person."

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Answer

You can use the word, "incredible" to mean not believable.  However, incredible also means great, so to avoid confusion we usually say "not credible".
http://learnersdictionary.com/definition/incredible

You could also use a synonym:  https://www.thesaurus.com/browse/incredible

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Hi Shia, 
It is possible to say “not a credible person” in English, but that has a meaning more like “not a person who can be believed as a source of information”.  In the context you give, I’d suggest the slightly different phrases “not a reliable person” or “not a trustworthy person”.

Answer

unreliable