One who's not in the habit of wearing a habit

Wiktionary Word definitions in Wiktionary
n. 1 A woman who is a layperson, one who has not taken a religious oath (such as becoming a nun). 2 by analogy, a woman who is not a professional in a given field

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
1520s, from lay (adj.) + woman ; probably modeled on layman .