Really really bad
(Other definitions for terrible that I've seen before include "Frightful" , "Like tsar Ivan IV" , "Extremely bad" , "What precedes twos < and follows child in France" , "Awful, dire" .)
'really really bad' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for abysmal that I've seen before include "Immeasurably bad" , "Really terrible" , "Very bad or bottomless" , "Bottomless - profound" , "Extremely bad - lamb, say (anag)" .)
(Other definitions for nasty that I've seen before include "Offensive or malicious" , "Highly repugnant" , "Obscene" , "Filthy" , "Mean" .)