Poisonous, noxious (5)
I believe the answer is:
toxic
I believe this is a double definition.
'poisonous' is the first definition.
(similar in meaning)
'noxious' is the second definition.
(associated in meaning)
(Other definitions for toxic that I've seen before include "word describing bad things" , "Harmful - cox it (anag)" , "Deadly - venomous" , "Poisonous, noxious" .)