gros plein de soupe

a fat person, fatty; (especially) one that seems clumsy or somewhat idle-looking

“Je crois que vous vous trompez, camarade, me répondit-il. Ce Petit-Blanc est un gros plein de soupe, impertinent, qui m’a l’air de faire plus de bruit que de besogne.”

From gros + plein + de + soupe, literally “fatty full of soup”, perhaps from the appearance children have after having eaten too much soup, later extended to be said of adults.