Green’s Dictionary of Slang

1. a story; also as v. to tell a tale.

2000		P. Howard Miseducation of Ross O’Carroll-Kelly (2004) 191: What’s the Jackanory?
2005		P. Howard Curious Incident of the Dog in the Nightdress 54: I peg it up [...] to see what the Jackanory is.
2009		T. Black Gutted 107: What’s the Jackanory with him and Rab Hart?
2016		A. Wheatle Crongton Knights 131: Bit and Jonah? Dancing? You’re not jackanoring me???!!!
2021		P Howard Braywatch I’m like, ‘Claire? What’s the Jack?’: .

2. a lie, i.e. a ‘tall story’.

1983		J. Sullivan ‘Thicker than Water’ Only Fools and Horses [TV script] Jackanory Jackanory.
1997		Share Slanguage.
2002		D. Shaw ‘Dead Beard’ at www.asstr.org 🌐 There was no way you needed to go into a court to tell jackanories about Lingers, you were his legal wedded boiler house.