What it names now
Standby is the English name for whatever is kept close enough to act at once. It can be a person: an understudy in the wings, a reserve officer, a traveler hoping a cancelled seat will open. It can be a thing: a basement generator, a spare recipe, a restaurant you trust when nothing else is open. It can be a state: troops held ready, a laptop in a low-power waiting mode, a radio operator listening for the next call. The fused spelling standby is now the usual noun, adjective, and adverb. The hyphenated stand-by still appears. The verb that produced them is still written as two words, stand by.
English did not import this form from French or Latin. It grew at home from a phrasal verb that had already been in the language for centuries.
A phrase before it was a noun
Late in the twelfth century, to stand by someone meant to stand near and watch. By the middle of the thirteenth century the same phrase also meant to wait, to remain beside, and to give support. Those older layers are still audible. Standing by a friend is loyalty. Standing by while harm is done is refusal to act, the same spatial idea that gives English bystander. Standing by for orders is readiness, not idleness.
As a command, stand by is recorded from the 1660s: hold yourself in readiness. Only after that military and nautical tone was familiar did speakers start using the phrase as a name for the person or thing that stays close in case of need.
Here the reference books diverge, and the disagreement is worth stating plainly. The Oxford English Dictionary’s earliest evidence for the noun is 1712, in a diary by E. Harrold; Merriam-Webster’s current entry follows that early-eighteenth-century date. Other historical dictionaries place the noun around 1790–1800 and treat an early sense as nautical: a vessel kept nearby for emergencies. A 1796 letter preserved among Nelson’s published dispatches calls a ship a stand-by in exactly that way. The two dates time different snapshots. The fused noun is already eighteenth-century. The sea image that later surveys often lead with is late eighteenth-century, not the invention of the word.
Two Germanic pieces
The origin of the word "Standby" is the verb stand plus the particle by, later treated as a single noun. Stand comes from Old English standan, a strong verb meaning to occupy a place, remain, stand firm, and resist. Proto-Germanic *standanan is the reconstructed source, with kin in Old Norse standa, Old Saxon standan, Old Frisian stonda, Gothic standan, and Old High German stantan. Later languages shortened the stem: Dutch staan, German stehen, Swedish stå. Behind the Germanic group sits the Indo-European root reconstructed as *sta-, to stand or to be firm. Latin stare and Greek histanai are distant relatives of that root, but English did not need them to build this compound.
By is the same word as the ordinary preposition. Old English had unstressed be- and stressed bi, from Proto-Germanic *bi, around, about, or near, itself a reduced form of Indo-European *ambhi-, around. Cognates include Dutch bij, German bei, and Gothic bi. In the phrasal verb the particle keeps its old spatial force: near at hand.
The noun is deverbal. English took a verb phrase and used it as a thing, the way it later did with other particles in words such as bystander. The usual plural is standbys, which keeps the second element intact instead of treating the word as an ordinary vowel-plus-y noun.
Close cousins of the idea, not of the fused spelling, appear in other Germanic languages. German beistehen and Dutch bijstaan both mean to stand by someone in the sense of help. The English phrase also travelled. In the Philippines, Tagalog istambay and Cebuano estambay, often shortened to tambay, come from stand by and now name a person hanging around with nothing particular to do. Waiting near at hand can look like duty or like loitering; both readings were already in the English verb.
How the sense moved
Once the noun existed, its work was to name a reserve. By 1801 it could mean a person who stands by another, a staunch adherent, the sort of ally later called an old standby. By 1861 it is recorded for something on which one can rely. The adjective, held near and ready, is dated to 1882. In the mid-twentieth century the word specialized again. By 1946, of people, it names a state of being ready for duty. By 1961, in civil aviation, it describes travel without a booked seat, waiting for a cancellation. Merriam-Webster dates the adverb, to fly standby, to 1971.
Radio and television later used the same reserve sense for a filler program kept ready if the scheduled one failed. Electronics borrowed the picture once more. A set left in standby is powered enough to wake at once, neither fully off nor fully at work. That technical sense is late, but it is faithful to the old spatial metaphor: the machine stands by.
An everyday use survives in old standby, a trusted recipe, method, or haunt. Here the word no longer points only to an emergency substitute. It points to the thing you return to because it has already stood by you.
The nautical image remains the cleanest illustration of the morphology. A ship that stands by is not the main actor in the scene; it stays within reach. English turned that posture into a noun, then into an adjective and an adverb, without changing the two Germanic pieces it was built from.
The date that clarifies the story
If one fact is worth carrying away, it is the split in the record. The fused noun is in print by 1712. The standby vessel that many etymologies lead with is cited from 1796. The later date did not coin the word. It shows how a native phrase for standing near became a name for whatever is kept close enough to save you.