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

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

Encyclopedia: Abysmal