a.

1. 怀疑的，疑惑的[F][（+about/of）][+wh-]
   I'm doubtful whether she's still alive.

   我怀疑她是否还健在。

2. 可疑的，令人生疑的
   a doubtful remedy

   疗效可疑的疗法

3. 含糊的，不明确的；难以预测的
   The future looks very doubtful.

   前途看来难以预测。

4. （名声等）不好的

adj.

1. uncertain.
2. not known with certainty.
   ▸improbable.
   ▸not established as genuine.