Green’s Dictionary of Slang

1. (US) a (pompous) fat person.

1939		‘Citadel Gloss.’ in AS XIV:1 Feb. 27/2: gutbucket, n. A pompous individual, usually corpulent.
2001		C. Brookmyre A Big Boy Did It and Ran Away 20: Would they rather open your neat, shiny briefcase than the forbiddingly grubby overnight bag of the eye-stingingly sweaty gut-bucket ahead of you.

2. (US) a toilet.

1942		Berrey & Van den Bark Amer. Thes. Sl.
1949		Monteleone Criminal Sl. (rev. edn) 110: gut bucket A chamber pot used in a prison cell.

3. (US) the belly.

1957

H. Ellison ‘Look Me in the Eye, Boy!’ in Deadly Streets (1983) 159: The man was [...] just ripe for a switch-blade in the gut bucket.