it's what he didn't say

When driving up to the Soo we came across this sight.
Martin harumphed and said "I could say something, but I won't."
"Did you have something you wanted to say?" I inquired.
"Not a thing, I can't think of a thing - here a pause - Nope, not a thing."
"Did you want to take that sign and strap it to my behind?"
"I didn't say that"
"No, you didn't but you are in trouble just the same"