to be screwed over on any sort of in any way. Use of word ranges from buying a used car to being cut off in traffic. It doesn't take much imagination to figure out it literally means to be fucked in the ass with a corncob.
Ben: "Hey Curtis, what did you get on your english essay?"
Curtis: I got--
Ben: I got an A!
Curtis: an F.
Curtis: The english teacher really corncobbed me on that one