mortise (slot cut into wood to receive a tenon); 4th earthly branch: 5
Depicts two knives side-by-side. Based on the original meaning "butcher's knife". The current meaning is a phonetic loan.
《說文》：“卯，冒也。二月萬物冒地而出，象開門之形。故二月為天門。，古文卯。”
butcher's knife