Green’s Dictionary of Slang

a trustworthy, knowledgable individual.

1849		Morn. Chron. (London) 27 Nov. 5/2: The measures by which they sell winkles and cockles [...] have all false bottoms [...] and can be inserted or slid out dexterously acclrding as the customer is a ‘jonnock’ or a ‘scaly’ cove.
1908		Magnet 29 Feb. 15: But I did think you were a real jannock; true all through.