1. in a way that causes fear or alarm; terrifyingly
2. extremely; very much
Usage: chiefly British; informal

Examples

• The monster looked frightfully scary in the dim light.
• She was frightfully worried about her missing cat.
• The old house creaked frightfully during the storm.
• He’s frightfully clever at solving puzzles.
• The weather has been frightfully cold this winter.
• The children were frightfully excited about the trip.
• The accident scene was frightfully disturbing.