Mistaken; illegal (5)
I believe the answer is:
wrong
I believe this is a double definition.
'mistaken' is the first definition.
(similar in meaning)
'illegal' is the second definition.
(thesaurus)
(Other definitions for wrong that I've seen before include "Mistaken" , "Treat unjustly" , "Erronious" , "Failing to come right" , "Injury - incorrect" .)