3rd earthly branch: 3-5 a.m., 1st solar month (4th February-5th March)
Originally written the same as 矢 (arrow). Later two hands (𦥑) were added to distinguish the characters.
《說文》：“寅，髕也，正月陽氣動，去黄泉欲上出，陰尚強，象宀不達，髕寅於下也。”