Green’s Dictionary of Slang

a foolish or comic person; thus (UK juv.) soppy date, an affectionate term of abuse.

1941		G. Kersh They Die with Their Boots Clean 75: I’ll instinctive you, you sloppy great Date, you!
1943		M. Harrison Reported Safe Arrival 11: Yer whistle, yer balmy date!
1959		I. & P. Opie Lore and Lang. of Schoolchildren (1977) 66: You’re a soppy date.