Meaning

noun a member of the Religious Society of Friends founded by George Fox (the Friends have never called themselves Quakers)

noun one who quakes and trembles with (or as with) fear

originFrom quake + -er; a name given to members of the Religious Society of Friends, supposedly by the magistrates Gervase Bennet and Nathaniel Barton, when George Fox "bade them tremble at the word of the Lord"; the term was previously applied to certain people who trembled or quaked during religious devotions.

Etymology from Wiktionary, CC BY-SA 3.0