when one eats so much corn, either being on the cob, from the can, or from any other source, and the corn shows up in the fecal matter, or poop, shit, turd. Now, for the poop to actually be called "cobbin'" is when it is filled with so much corn that it looks like corn on the cob
"hey man! I just went cobbin'!"
"you ever been cobbin'?"
"that shit was straight cobbin'"