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means The informal title for the wife (or, by extension, leading hostess) of a head of state or governor, who takes on ceremonial and social duties with no official office or pay.

from American in origin, built simply from 'first' (foremost in rank) plus 'lady.' It's often said to have been coined for Martha Washington or popularized at Dolley Madison's funeral eulogy in the 1870s, but the phrase actually drifted into use gradually across the 1800s as newspapers needed something grander than 'the president's wife.' By the early 20th century it had hardened into the fixed title we use today.