1798, "of or belonging to the Isle of Man," between England and Ireland, earlier Manks (1620s), metathesized and contracted from Maniske (1570s) "of the Isle of Man," from Old Norse *manskr, from Man (from Old Irish Manu "Isle of Man") + suffix -iskr "ish." As a noun, 1680s as "native or inhabitant of Man," from 1670s in reference to the Celtic language spoken there (extinct since mid-20c).  Manx cat, without a tail, is attested by 1843; the natural mutation arose among cats there and took root in a limited gene pool.

updated on November 19, 2018

Definitions of Manx from WordNet
Manx (n.)
the ancient Gaelic formerly spoken on the Isle of Man; the language is sometimes used on ceremonial occasions;
Manx (n.)
a short-haired tailless breed of cat believed to originate on the Isle of Man;
Synonyms: Manx cat
Manx (adj.)
of or relating to the Isle of Man or its inhabitants or their language;
Etymologies are not definitions. From, not affiliated with etymonline.