a.

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

   我懷疑她是否還健在。

2. 可疑的，令人生疑的
   a doubtful remedy

   療效可疑的療法

3. 含糊的，不明確的；難以預測的
   The future looks very doubtful.

   前途看來難以預測。

4. （名聲等）不好的

adj.

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