When someone speaks of "holding down the fort," it basically means keeping an eye on things temporarily while the person in charge is away. The expression seems rather nonsensical, though; a fort is a large, solid building constructed as a stronghold. A person in an actual fort might need to hold up the fort (or its walls) if it came under attack, but you don't hold down an inanimate object that is too heavy for the wind to blow away. So where does the term originate?
Hold the fort (British, American & Australian) also hold down the fort (American):
- to be left in charge of a situation or place while someone is away. Someone had to stay at home and hold the fort while my mother was out.
(Cambridge Idiom Dictionary)
According to the Phrase Finder:
The correct phrase is "hold the fort" - there's no "down".
Since the Middle Ages "hold" in a military context has meant, "to keep forcibly against an adversary; defend; occupy". If the commander of a fort decided to take some of his forces to make a foray against the enemy, he would always have to leave some of his men in charge of a reliable officer to hold the fort against any possible attack while they were away. (VSD)
"Hold down the fort" is a variation . The original use of the phrase "hold the fort" was a military order wired by Gen. William Tecumseh Sherman in 1864 to Gen. John M. Corse at Allatoona during the Civil War. "Records show that the actual words had been 'Hold out, relief is coming,' but 'fort' is what caught on and was further popularized when it was made the refrain of a gospel song by Philip Paul Bliss." From "Facts on File Dictionary of Cliches," second edition, edited by Christine Ammer, Checkmark Books, New York, 2006. Page 202.
I accept that this incident is what popularised the phrase, but it can't possibly have been the original use! English-speaking people have been holding forts, and ordering other people to hold forts, for close on a millennium. (VSD)
Using the google on books finds a "down" usage from 1951. It took Wilson Follett and Jacques Barzun fifteen years to contemne this phrase in their *Modern American Usage: A Guide" in 1966, saying "Many unschooled in the lore of battle hold an odd idea of forts. For more than a century, the idiom, commonly figurative, has been to hold the fort -- that is, to retain possession of a place against all threat of contention.... Those who have taken to saying hold down the fort would never say hold down an odd idea of forts. Which seems to miss the point, since holding an idea differs significantly from holding a fort.
The Random House Historical Dictionary of American Slang notes the use, dating from the late 19th Century, of "hold down," meaning to occupy a place, and A Dictionary of Slang and Unconventional English records the use from the same period with land claims. And no one finds strange the locution "hold down a job." Using "down" may be a corruption of the original, but I don't see how it's based on a mishearing that would make it an eggcorn.
It's "hold the fort," which makes sense militarily.
Keep possession of (something), typically in the face of a challenge or attack: the rebels held the town for many weeks
Adding down to the idiom "hold the fort" is an example of an eggcorn.
a word or phrase that is a seemingly logical alteration of another word or phrase that sounds similar and has been misheard or misinterpreted, as 'old wise tale' for 'old wives' tale'.
So if "down" is there in your experience of common usage, it's just a misinterpretation of the original military idiom, or a misunderstanding of how to use the verb hold in this sense, though it's not necessarily wrong.