Sunny Jim, n. — Green’s Dictionary of Slang

Sunny Jim n.

[slogans for Force whole wheat breakfast food, e.g.: ‘High o’er the fence leaps Sunny Jim / “Force” is the food that raises him’ coined 1902]

a general term of address, esp. affectionate.

[1902		Sheffield Dly Teleg. 6 Dec. 3/6: [advt] Jimp Dumps was a most unfriendly man [...] Till ‘force’ one day was served to him — Since then they call him ‘Sunny Jim’ ].
[1903		Eve. Teleg. 24 Sept. 3/3: Genesis of ‘Sunny Jim’. Few people who smile at the agility of ‘Sunny Jim,’ as depicted by the poster of an enterprising food company, know that he owes his existence to a girl of 18].
1909		Dundee Courier (Angus, Scot.) 27 Sept. 6/7: [pic. caption] ‘Sunny Jim’ Young, Stalwart Celtic Half.
1947		N. Streatfeild Grass in Piccadilly 111: We want a proper Sunny Jim for a baby.
1978		S. King Stand (1990) 161: In the back of the cruiser, Sunny Jim.
1990		H. Kureishi Buddha of Suburbia 24: London, the Old Kent Road [...] It was wet and foggy; people called you ‘Sunny Jim’.