In earlier times, a corncob was used by some for personal hygiene concluding a visit to the outhouse. To say that one is "rough as a cob" is to note their abrasive manner, or unpolished style of approach usually in regard to communications or actions.
You know John ain’t got no smarts, he’s as rough as a cob.
by NC get it right September 28, 2006
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Mighty unpleasant.

From the rough surface of a corn cob used to wipe one's backside.
"Hey Elmore, the wheel done fell off mah baler."

"Oh man, Ermal, that's rough as a cob."
by The Wog Whomper May 1, 2005
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