Used predicatively: in the least favourable or advantageous position; (now) especially least well provided with money or other resources.
In attributive use. Usually with hyphen. Least favourably placed or advantaged; especially least wealthy or well-to-do; poorest.
With the and plural concord. Those who are in the least favourable or advantageous position; especially people who are worst off financially, considered as a class.
Late 18th century; earliest use found in Peter Beckford (1740–1811), dog breeder and writer on hunting. From worst + off.