a.

1. 可怕的
   I was awakened by a frightful clap of thunder.

   我被一声可怕的雷声惊醒。

2. 难看的，丑的
3. 【口】非常的；讨厌的；不快的
   We had a frightful journey.

   我们作了一次不愉快的旅行。

adj.

1. very unpleasant, serious, or shocking.
2. informal terrible; awful:
   her hair was a frightful mess.