The Collaborative International Dictionary of English v.0.48:

Deleterious \Del`e*te"ri*ous\, a. [LL. deleterius noxious, Gr.
   dhlhth`rios, fr. dhlei^sqai to hurt, damage; prob. akin to L.
   delere to destroy.]
   Hurtful; noxious; destructive; pernicious; as, a deleterious
   plant or quality; a deleterious example. --
   Del`e*te"ri*ous*ly, adv. -- Del`e*te"ri*ous*ness, n.
   [1913 Webster]