WordWeb Online

1. Very great; limitless
   "abysmal misery"; "abysmal stupidity"
    
2. Resembling an abyss in depth; so deep as to be unmeasurable
   "the abysmal depths of the ocean"
   - abyssal, unfathomable
    
3. Very bad
   "it was an abysmal performance"

See also: abysm, deep, immeasurable, immensurable, immensurate, unmeasurable, unmeasured