You know how, in the movies, old men sit around at a barbershop telling each other really elaborate, funny stories that couldn't possibly be true? That's cock-and-bull, and it is a craft to be mastered. Everybody loves a good story, and when you can tell one in good company that evokes laughter and respect, you're doing it right.
These tales are usually spun by males, though the greatest cock-and-bull I ever heard came from a gal who smoked two packs a day bartending at the neighborhood bar. She'd be real quiet while everyone else told their stories and, when it seemed a winner had been chosen, she'd wipe off the bar with a dirty white towel, like she was clearing the way for something big. She always made her stories up on the spot, too. Spontaneity is as much a part of the craft as facial expression and vocal inflection.
For years, I thought it was "cockembull stories." My great-grandfather would take us upstairs on Christmas Eve and tell us these stories while Santa left gifts at the tree below us. We would huddle in a nice down comforter and listen to him ramble on and on about a dog in a wheatfield, and the time he got his brother Jimmy covered in skunk spray, and so on. I loved to hear cockembull. Thought it was an Indian word; like we were at a pow-wow and the great chieftan was dancing around the fire in a display of feathers and power. It's amazing how we invent our own understanding.
Note: I was unable to find an origin for the phrase "cock and bull." Some people quote a story about an English crossroads where the Cock Inn and Bull Inn were located. This is a great example of folk etymology, but not fact. A more reasonable explanation might be the usage of animals in fictional tales, like the work of the Brothers Grimm