Green’s Dictionary of Slang

(US) thick, clinging mud.

1881		Norton Courier (KS) 2 June 4/2: [T]here being no stony or sandy prairie, no ‘gumbo,’ hard-pan or alkali.
1894		Century Illus. Mag. Jan. 453/1: Do you know what gumbo is? [...] When wet it is the blackest, stickiest most India-rubber-like mud that exists on earth.
1919		S. Lewis Free Air 4: The car shot into a morass of prairie gumbo‚—which is mud mixed with tar, fly-paper, fish glue, and well-chewed, chocolate-covered caramels 14: [T]hey started out of Minneapolis in a mist, and as it has been hinted, they stopped sixty miles northward, in a rain, also in much gumbo.
1947		W.D. Overholser Buckaroo’s Code (1948) 74: I sure hate to walk through gumbo when it’s wet [...] Balls up your feet something awful.
1984		A. Sample (con. late 1950s-early 1960s) Racehoss 202: I sank my hoe blade deep as I could each time I dug it into the soft, black gumbo.