When you are unsure about something and are not certain about its validity, you might use the phrase "not certainly." However, there are several alternative phrases that you can use to convey a similar meaning. Some examples include "not definitively," "not absolute," "not conclusive," "not confident," "not convinced," "not sure," "not reliable," "not dependable," "not decisive," and "not specific." These different synonyms allow you to be more specific about how you feel about a certain situation, and they give you more options to choose from when expressing your uncertainty about something.