Green’s Dictionary of Slang

(mainly Scot.) used as a term of address to a person whose actual name one does not know.

1915		‘Ian Hay’ First Hundred Thousand (1918) 157: A company where practically every man is addressed either as ‘Jock’ or ‘Jimmy’.
1920		Ade Hand-made Fables 47: Old moonfaced Joe got his Promotion just when the wise Jimmies got the Sidewalk.
2000		Indep. on Sun. Rev. 30 Apr. 45: Men whose personal philosophies consisted, roughly, of ‘See you Jimmy’.