A great big, huge fat white line, guarenteed to bring a grin to your face.
"Hey Gumbo, I've lined up loads of John Candy's - come and get yours"
An expression used to insult men with tiny penises, with great sarcasm, as no-one has got a cock the massive size of Gumbo.
Commonly used phrases include:
Your cock is tiny compared to Gumbo's Knob.
Gumbo's Knob is just the best.
I absolutely need something as huge as Gumbo's Knob.