Abbreviation of 尽日 (じんじつ). Used as a suffix in month-end expressions like 月末 (げつまつ) or 三月尽 (さんがつじん). Rare as a standalone noun.
See also: 尽日 (じんじつ)
The last day of March is the 31st.
尽 is an old way to say the end of the month; nowadays 月末 is more common.