Green’s Dictionary of Slang

‘I’ll be with you at once’, ‘I won’t be long’; thus derisive phr., aimed at a slow person, coming? so is Christmas.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Coming! so is Christmas. said of a person who has long been called, & answers, Coming!
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn).
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.