The Collaborative International Dictionary of English v.0.48:

Amish \Am"ish\, a. [Written also Omish.] (Eccl. Hist.)
   Of, pertaining to, or designating, the followers of Jacob
   Amman, a strict Mennonite of the 17th century, who even
   proscribed the use of buttons and shaving as "worldly
   conformity". There are several branches of Amish Mennonites
   in the United States. A branch having particularly strict
   adherence to the Amish principles are called Old Order
   Amish
   [Webster 1913 Suppl. +PJC]