ixcuahuac.

a flint knife used to cut open the breast of a captive for extracting the heart in a human sacrifice, such as upon drilling the fire at the start of a new 52-year cycle
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 7 -- The Sun, Moon, and Stars, and the Binding of the Years, Number 14, Part 8, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1953), 28.