frighteningly

fright·en  (frtn)

v. fright·ened, fright·en·ing, fright·ens

v.tr.

1. To fill with fear; alarm.

2. To drive or force by arousing fear: The suspect was frightened into confessing.

v.intr.

To become afraid.

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frighten·er n.

frighten·ing·ly adv.

Synonyms: frighten, scare, alarm, terrify, terrorize, startle, panic
These verbs mean to cause a person to experience fear. Frighten and the less formal scare are the most widely applicable: "The Count's mysterious warning frightened me at the time" (Bram Stoker). The angry dog scared the small child.
Alarm implies the often sudden onset of apprehension: Her sudden weight loss alarmed her doctor.
Terrify implies overwhelming, often paralyzing fear: "It is the coming of death that terrifies me" (Oscar Wilde).
Terrorize implies intimidation and sometimes suggests deliberate coercion: "The decent citizen was terrorized into paying public blackmail" (Arthur Conan Doyle).
Startle suggests a momentary shock that may cause a sudden, involuntary movement of the body: The clap of thunder startled us.
Panic implies sudden frantic fear that often impairs self-control and rationality: The realistic radio drama panicked the listeners who tuned in after it had begun.

Thesaurus

Adv.

1.

frighteningly - in an alarming manner; "the disturbing thing about the Minister's behavior is that far from being artificial, it too often rings frighteningly true" scarily