okay, here they are (vote for your favorite bluff):

1. A surgical procedure involving removal of growths blocking breathing passages and sinuses.
1a. formal term for the phrase "ripping you a new one"

2. A surgical procedure to remove four hemorrhoids at the same time.

3. cuttting hairs, into four bits or lengths. Gimme the prize now

4. an ingenious use of semi-recursion in electronic chip design where 4 (tetra) gates (from greek pyli) are made to do the work of 8 (octo).

5. An exploratory surgical procedure involving four incisions in the pelvic area.

6. The removal of a third limb.

7. The surgical removal of one of a cow's four stomachs.

8. Surgical removal of a hair follicle tumor [tetrapylotoma] which contains cells descended from all three embryonic cell lines: endoderm, mesoderm, and ectoderm.

9. the surgical replacement of a missing leg

10. the surgical removal of the fourth rib

11. surgical alteration of the pyloric region of the stomach by means of four parallel incisions in the inner wall

12. A surgery in which the appendix, gall bladder, spleen, and pancreas are removed.

13. In veterinary medicine, treatment of the bovine digestive tract.

[at least five people knew the definition; three of those submitted bluffs also; one was a wise-guy and reworded it.]