Green’s Dictionary of Slang

(UK Und.) a blow; usu. as dub o’ the hick, a blow on the head.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Dub o’ th’ Hick a Lick on the Head.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn).
1811		Lex. Balatronicum.
1812		Spirit of Irish Wit 102: He gives him a dub with his daddle upon de snotter-box and brought de Claret about his mug.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1859		Matsell Vocabulum 28: dub o’ the hick A blow on the head. The copper tipt the dromedary a dub o’ the hick with his drum-stick.
1895		Crowe Aus. Sl. Dict. 25: Dub o’ the Luck [sic], a blow on the head.