say it: FRAHY-tuh-ning-lee
frighteningly is an adverb meaning "in an alarming manner". It carries one recorded sense.
in an alarming manner
“the disturbing thing about the Minister's behavior is that far from being artificial, it too often rings frighteningly true”
Definitions, synonyms and semantic relations for frighteningly come from Princeton University’s WordNet. The pronunciation comes from the CMU Pronouncing Dictionary. Commonness is measured against a corpus of everyday English. Full sources and licences.