Douglas Harper's Etymology Dictionary
"person who never dies but becomes senile and useless," 1726, from "Gulliver's Travels," a made-up word.
Wikipedia
In Jonathan Swift's novel Gulliver's Travels, the name struldbrug is given to those humans in the nation of Luggnagg who are born seemingly normal, but are in fact immortal. However, although struldbrugs do not die, they do nonetheless continue aging. Swift's work depicts the evil of immortality without eternal youth.
They are easily recognized by a red dot above their left eyebrow. They are normal human beings until they reach the age of thirty, at which time they become dejected. Upon reaching the age of eighty they become legally dead, and suffer from many ailments including the loss of eyesight and the loss of hair.
Struldbrugs were forbidden to own property:
As soon as they have completed the term of eighty years, they are looked on as dead in law; their heirs immediately succeed to their estates; only a small pittance is reserved for their support; and the poor ones are maintained at the public charge. After that period, they are held incapable of any employment of trust or profit; they cannot purchase lands, or take leases; neither are they allowed to be witnesses in any cause, either civil or criminal, not even for the decision of meers (metes) and bounds.
Because:
Otherwise, as avarice is the necessary consequence of old age, those immortals would in time become proprietors of the whole nation, and engross the civil power, which, for want of abilities to manage, must end in the ruin of the public.
Usage examples of "struldbrug".
Not a single Struldbrug was less than one hundred and fifty-four years of age, by Club law, and the age requirement went up one year for every two that passed.
Gulliver said they had a law, no Struldbrug could keep his money after he was a hundred.
To me he is, like the greater number of classics in all ages and countries, a literary Struldbrug, rather than a true ambrosiafed immortal.
They stood there for a long moment, watching the cloud drift out to sea, an insubstantial monument to the suicide of the Struldbrugs, but the only monument they would ever have.