Green’s Dictionary of Slang

(Aus./US) a clock.

1935		A.J. Pollock Und. Speaks.
1944		Maurer & Baker ‘“Aus.” Rhyming Argot’ in AS XIX:3.
1946		St Vincent Troubridge ‘Some Notes on Rhyming Argot’ in AS XXI:1 Feb. 46: hickory dock. A clock. (Origin uncertain, American or English.) Probably American in this form.
1953		Berrey & Van den Bark Amer. Thes. Sl. (2nd edn).
1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxv 6/2: hickory dickory: Clock.
1977		S. Berkoff East in Decadence and Other Plays (1985) 64: I was thinking of that bird which was making me very anxious about the Hickory-Dickory.