An early use of "frock" (15th century) referred to the long habit characteristically worn by monks. Through the centuries, frock came to describe various loose garments of some length. The "frock coat," which was a long-skirted garment coming almost to the knees, became a popular fashion for men in the early 19th century and was quickly adopted for military uniforms. It is feasible that the frock coat was so called because the length was reminiscent of earlier clothing articles.
There is an alternative explanation for the term "frock coat." An ornamental closure, called a "frog," which consisted of a spindle-shaped button passing through a loop of material or braid, was typical on military uniforms at least as early as the mid-18th century. The expression could have evolved from frog coat to frock coat.
Our assumption is that the current usage of "frocking" is in some way related to the officer's frock coats. The verb form "to frock," relating originally to the monk's cloak, meant "to invest with priestly office or privilege." Perhaps this idea of establishing position by the donning of a particular garment is resurrected here. Another explanation for the link between the term and the practice is that the early undress uniform for a midshipman was a short coat whereas that for a lieutenant was a frock coat. When a midshipman was appointed to act in the capacity of a lieutenant, he wore the uniform of the latter. In this instance, it could be said he was "frocked."
Regarding the practice of frocking itself, there are various instances in Navy Regulations at least as early as 1802 of personnel assuming the uniform of the next higher rank, not necessarily with higher pay, when appointed by proper authority to assume the duties and responsibilities of that rank prior to actual promotion. These appointments were sometimes temporary as when it was necessary rapidly to swell the ranks during war time, especially in the Civil War. Other instances concerned a commander's need to fill an unforeseen vacancy for which there was no one of equal rank. In this case, a junior who was qualified for promotion would assume the uniform and duties pending approval by the Navy Department.
The practice that we currently refer to as frocking has been in common usage in one form or another throughout the Navy's history. There does not seem to be a definite point however at which the practice was first established. The original use of the term "frocking" to describe the current policy is also unclear, but probably is of recent vintage as it does not appear in earlier documents (18th-19th century). The 1974 version is the earliest Bureau of Naval Personnel Manual to contain the policy itself and refer to it as "frocking."