Green’s Dictionary of Slang

vapid, naïve, esp. romantic.

1911		L. Chevalier ‘Getting into Society’ Variety Stage Eng. Plays 🌐 She’s soupy in the noodle.
1929		J. Lait Broadway Melody 69: Stewie’s a soupy safe blower that never got further than smoke and noise. But Warry’s a soapy cold-fingered operator that untwists combinations.
1930		L. Lower Here’s Luck 110: ‘I’m looking for a young man,’ I said, ‘tall, inclined to be fair, rather soopy expression’.
1949		Wodehouse Mating Season 37: She is squashy and soupy.
1969		P. Gilliatt in New Yorker XLIII 36/3: He remembered a girl he had known once who used to say ‘franchement’ whenever she meant ‘frankly’ — rather a soupy girl, and far from frank, but consoling.