Green’s Dictionary of Slang

cod-on n.

[cod v. (2)]

(Irish) a practical joke.

[UK]D. Marcus ‘Beginnings’ in Honest Ulsterman 37-41 9: Father [...] had warned me about ‘cod-ons’ – the apprentice sent traipsing from shed to shed in search of ‘the rubber-headed hammer’ or ‘bucket of blue steam’ .