The Collaborative International Dictionary of English v.0.48:

Skag \Skag\, n. (Naut.)
   An additional piece fastened to the keel of a boat to prevent
   lateral motion. See Skeg.
   [1913 Webster]

WordNet (r) 3.0 (2006):

skag
    n 1: street names for heroin [syn: big H, hell dust, nose
         drops, smack, thunder, skag, scag]