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deleterious

adjective
 
 
/ˌdel.ɪˈtɪə.ri.əs/ US  /-ˈtɪr.i-/ formal

Definition

harmful: These drugs have a proven deleterious effect on the nervous system.Dangerous and harmfulSerious and unpleasantNot attractive to look atInformal words for bad deleteriously
 
 
/-li/ adverb
(Definition of deleterious adjective from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)
Focus on the pronunciation of deleterious