"as plain as a quaker"
by cm

The Religious Society of Friends, or Quakers, is a Christian movement, born out of 17th century dissenters. They played an important role in the improvement of social conditions for the poor and the abolition of the slave trade.

Quakers were known for their plain dress and minimalist meetings, with no leader or programme and little music.  The name quaker was adopted because of their message that people should tremble at the Word of God.