History of the Frock
Originally, a frock was a loose, long garment with wide, full sleeves, such as the habit of a monk or priest, commonly belted. (This is the origin of the modern term defrock or unfrock, meaning "to eject from the priesthood").
From the sixteenth century to the early twentieth century, frock was applied to a woman's dress or gown, in the fashion of the day, often indicating an unfitted, comfortable garment for wear in the house, or (later) a light overdress worn with a slip or underdress.
Frock (especially in the phrase 'short frock') is also a child's dress or light overdress.
Frock's older meaning is retained in dialect words for men's outer garments:
From the seventeenth century on, a frock is a thigh- or full-length loose outer garment worn by shepherds, workmen, and farm workers, generally of heavy linen (now usually called a smock-frock).
A frock is a dense knitted overgarment worn by sailors and fishermen, as guernsey frock, jersey frock (now usually simply guernsey and jersey).