The Probert Encyclopaedia of Costume

SMOCK

A smock was in mediaeval times a loose, knee-length or calf-length garment with a close gathered yoke, made of cotton or linen and worn by women under their gowns. In the 18th century the smock developed into a loose, yoked, shirt-like outer garment often with long sleeves, worn by farm workers. By the 20th century a smock was a yoked shirt or dress often worn as maternity wear, or as a protective overall.
Research Smock