In Gulliver's Travels, what is the purpose of the Brobdingnagians calling Gulliver a "lusus naturæ?"
Gulliver is extremely small compared to the Brobdingnagians, and so their scientists determine that he is something beyond their studies and beyond their comprehension of science.
They would not allow me to be a dwarf, because my littleness was beyond all degrees of comparison... After much debate, they concluded unanimously, that I was only relplum scalcath, which is interpreted literally lusus naturæ...
(Swift, Gulliver's Travels, eNotes eText)
"Relplum Scalcath" is the invented Brobdingnagian language, and translates to the Latin "lusus naturæ," or "freak of nature." This was used by scientists in earlier times to denote anything that was out-of-the-ordinary, or strange compared to the standard and known plants, animals, and minerals of science. By defining Gulliver -- an ordinary man of real-life England -- as something strange and unknowable, the Brobdingnagian scientists show that they are either unable or unwilling to entertain the thought that they themselves could be the extraordinary ones. It also allows them to avoid defining Gulliver as a person by their own standards, and so he does not have the same rights as a Brobdingnagian citizen.