punji stick

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A very sharp bamboo stake that is concealed at an angle in high grass, in a hole, or in deep mud, often coated with excrement, and planted to wound and infect the feet of enemy soldiers.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A basic weapon in the form a sharp spike in the ground, normally made from wood or bamboo and often covered in feces or other biohazardous material as a means to cause infections or disease.