Similar words: frightening, brightening, tightening, lightening, straightening, frighten, frighten off, frightened. Meaning: adv. in an alarming manner.
(31) Roald Dahl was of course frighteningly good at it because he let the child look at the fallibility (or worse) of the adults who have a duty of care.
(32) As the sparks died away, the sky seemed darker than ever , even frighteningly black.
More similar words: frightening, brightening, tightening, lightening, straightening, frighten, frighten off, frightened, frighten away, threateningly, brighten, self-righteous, sickeningly, fright, affright, righteous, benighted, frightful, tighten, lighten, unrighteous, righteously, heighten, stage fright, frightfully, lighten up, enlighten, tightened, tighten up, righteousness.