Frock has been used since Middle English

Middle English

Middle English is the stage in the history of the English language during the High and Late Middle Ages, or roughly during the four centuries between the late 11th and the late 15th century....

 as the name for an article of clothing

Clothing

Clothing refers to any covering for the human body that is worn. The wearing of clothing is exclusively a human characteristic and is a feature of nearly all human societies...

 for men and women (see also clothing terminology

Clothing terminology

Clothing terminology comprises the names of individual garments and classes of garments, as well as the specialized vocabularies ofthe trades that have designed, manufactured, marketed and sold clothing over hundreds of years....

).
Originally, a frock was a loose, long garment with wide, full sleeve

Sleeve

Sleeve is that part of a garment which covers the arm, or through which the arm passes or slips. The pattern of the sleeve is one of the characteristics of fashion in dress, varying in every country and period...

s, such as the habit

Religious clothing

-Christianity:Vestments are liturgical garments and articles associated primarily with the Christian religions, especially the Latin Rite and other Roman Catholics, Eastern Orthodox, Anglicans, Methodists, and Lutheran Churches...

 of a monk

Monk

A monk is a person who practices religious asceticism, living either alone or with any number of monks, while always maintaining some degree of physical separation from those not sharing the same purpose...

 or priest

Priest

A priest is a person authorized to perform the sacred rites of a religion, especially as a mediatory agent between humans and deities. They also have the authority or power to administer religious rites; in particular, rites of sacrifice to, and propitiation of, a deity or deities...

, commonly belted. (This is the origin of the modern term defrock or unfrock, meaning "to eject from the priesthood

Clergy

Clergy is the generic term used to describe the formal religious leadership within a given religion. A clergyman, churchman or cleric is a member of the clergy, especially one who is a priest, preacher, pastor, or other religious professional....

").

The term has been continually applied to various types of clothing, generally denoting a loosely fitted garment:

• From the sixteenth century to the early twentieth century, frock was applied to a woman's dress or gown
  Gown
  A gown is a loose outer garment from knee- to full-length worn by men and women in Europe from the early Middle Ages to the 17th century ; later, gown was applied to any woman's garment consisting of a bodice and attached skirt.A long, loosely-fitted gown called a Banyan was worn by men in the 18th...
  
  , 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
  Slip (clothing)
  Slip is a woman's undergarment worn beneath a dress or skirt to help it hang smoothly and to prevent chafing of the skin from coarse fabrics such as wool...
  
   or underdress.
• From the seventeenth century on, a frock is a thigh- or full-length loose outer garment worn by shepherd
  Shepherd
  A shepherd is a person who tends, feeds or guards flocks of sheep.- Origins :Shepherding is one of the oldest occupations, beginning some 6,000 years ago in Asia Minor. Sheep were kept for their milk, meat and especially their wool...
  
  s, workmen, and farm workers in Britain
  United Kingdom
  The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
  
  , generally of heavy linen
  Linen
  Linen is a textile made from the fibers of the flax plant, Linum usitatissimum. Linen is labor-intensive to manufacture, but when it is made into garments, it is valued for its exceptional coolness and freshness in hot weather....
  
   with a broad flat collar
  Collar (clothing)
  In clothing, a collar is the part of a shirt, dress, coat or blouse that fastens around or frames the neck. Among clothing construction professionals, a collar is differentiated from other necklines such as revers and lapels, by being made from a separate piece of fabric, rather than a folded or...
  
  , now usually called a smock-frock
  Smock-frock
  A smock-frock or smock is an outer garment traditionally worn by rural workers, especially shepherds and waggoners, in parts of England and Wales from the early eighteenth century...
  
  .