What is a frock?
From the 16th century to the early 20th 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 under-dress.
In British English and in Commonwealth countries the word may be used as an alternative term for a girl's or woman's dress. In Australia it is frequently used this way, with the phrase "to frock up" meaning to wear a formal dress or gown for a special occasion. This term was often used in Old English & early American literature.
The word frock may not be as common today as it was in the past, though it's a great way to refer to a dress. You can also call a monk's loose, long-sleeved garment a frock. The word's origin is Germanic, and it comes directly from the French word 'froc', "a monk's habit."
In contemporary times, a 'frock' may still designate a woman's, girl's, or child's dress or light overdress. Typically, girls and women wear frocks to formal events like weddings and fancy parties.