Questionable (adjective)

1. Open to doubt or skepticism.
2. Susceptible to being questioned or challenged.
3. Of doubtful quality or authenticity.

Origin:

From the latin word questionare meaning "to ask" or "to question".

Examples:

1. The validity of the research is questionable.
2. The authenticity of the painting is questionable.
3. The company's financial practices are questionable.
4. His motives are questionable.
5. The evidence presented in the trial was questionable.