Unfortunately we don't know what issue they thought the dictionary might be relevant to. I can imagine cases where it would not be appropriate to let them look in a dictionary. For instance, if there is an issue what the "man in the street" would have understood a word or sentence to mean, then the jury, as "men in the street" (OK, in the jury room, but you know what I mean) need to answer that from their own knowledge rather than looking it up in the dictionary.

As an example (albeit of a phrase not a word) in one of the last capital cases in the UK the jury had to decide what the (unarmed) defendant meant when he shouted "let him have it" to his co-defendant, who was pointing a gun at a policeman. Did he mean "give the policeman the gun" or did he mean "shoot the policeman".

No way a jury should be allowed a dictionary in that sort of case. They have to decide based on what they saw and heard on court.