9th calendar sign
壬 is primarily the ninth sign in the traditional Chinese sexagenary cycle calendar. Its standalone meaning is limited to this calendrical role, and it does not carry broader conceptual senses.
壬 is an ancient pictograph whose original meaning is uncertain. It was later borrowed as the ninth celestial stem in the sexagenary cycle. The modern form visually contains a slanted stroke over 士, but the historical development is not clearly tied to these components.
The top slanted stroke looks like a marker pointing to the bottom 士 (scholar). Imagine a scholar being assigned the ninth position in a calendar lineup, so 壬 is the ninth sign.
For ニン, picture a ninja holding the ninth calendar sign: ninja -> ニン, and the ninja is the ninth member of the team.
9th in rank; ninth sign of the Chinese calendar
Water Horse (19th term of the sexagenary cycle, e.g. 1942, 2002, 2062)
Imo Incident (riot in Seoul, Korea; July 23, 1882)
Water Rat (49th term of the sexagenary cycle, e.g. 1912, 1972, 2032)
Water Tiger (39th term of the sexagenary cycle, e.g. 1962, 2022, 2082)
Water Dog (59th term of the sexagenary cycle, e.g. 1922, 1982, 2042)
Brassica campestris var. laciniifolia (variety of wild mustard)
Water Monkey (9th term of the sexagenary cycle, e.g. 1932, 1992, 2052)
Jinshin revolt (671 CE)
Water Dragon (29th term of the sexagenary cycle, e.g. 1952, 2012, 2072)
Japanese invasions of Korea (1592-1598); Imjin war