A small hound, still in the language
In modern English a beagle is a compact scenthound: short in the leg, smooth in the coat, often marked in black, white, and tan, and long associated with hunting hare. The same dog is now as likely to live as a household companion as to run in a pack. Hunters who go out on foot with those packs still use a matching verb, to beagle.
The name is older than the standardized breed. English already had it in the fifteenth century. What it first named is clear: a small hare hound that hunted by scent. Where the name itself came from is not. Standard dictionaries still mark the etymology as unknown or only possible.
English by the late 1400s
The earliest recorded forms are Middle English begle and begel, with a plural begles. Merriam-Webster dates the noun to the fifteenth century; other desk dictionaries cluster the first uses around 1490 to 1500. A late-medieval romance, The Squire of Low Degree, dated around 1475 with some uncertainty, already puts begles in a hunting scene beside raches, the larger running hounds of the period. Later sixteenth-century spellings include begele before the modern beagle settles in.
Those early spellings matter for one hard phonological reason. The word has a hard g. In that position an inherited Old English g would normally have palatalized, so a native Old English pedigree is unlikely. The name looks borrowed into Middle English, or reshaped after the Old English period, rather than handed down unchanged from the earliest English.
The origin of the word beagle is therefore a problem of contact and analogy, not of a transparent native compound. English speakers have treated it as a simple noun ever since. There is no living prefix or suffix in the modern form to unpack.
The French loudmouth
The account printed most often looks across the Channel. Old French and Middle French had a compound of beer or bayer, to gape or open wide, and gueule, the mouth or throat of an animal, from Latin gula, the same Latin noun that later gives English gullet. Put together, bee gueule or beegueule named a noisy person, a shouter, someone who keeps the mouth open. Modern French bégueule survives, though the insult has drifted toward prude.
The attraction of that etymology is semantic as much as formal. An open throat is an easy nickname for a creature that gives tongue. Historical dictionaries note that Godefroy recorded Old French beegueulle for a noisy shouting person, and they allow that such an epithet might have been applied to a hound.
They also record the objection that stops the story from becoming a fact. There is no evidence that French speakers used that insult as a dog name. The later French word for the breed, bigle, was taken from English, not given to it. The later traffic in the breed name runs from English into French and into Dutch, Spanish, Polish, Finnish, and other languages that simply borrowed the English word with the dog.
The gape verb behind the French compound is the same family as English bay in the architectural sense, an opening, from a verb meaning to yawn or gape. That is a different word from the bay of hounds, the hunting cry, which has its own history. The French nickname is about a gaping mouth, not about an echoic howl, even if a noisy hound would fit the insult after the fact.
Other guesses, none proven
An older line of English dictionaries, including Webster, floated a Celtic source. Irish and Scottish Gaelic beag means small, from Old Irish becc, and Welsh bach is compared on the same grounds. A word for small would suit a little hound, but the match is of sense more than of form, and recent etymological handbooks do not treat the Celtic link as established.
Another proposal keeps the word inside English. On this view beagle is a reshaping of Middle English bedel, the ancestor of beadle, in the sense of a constable or officer who hunts people down. A change of d to g before l is a known English pattern; dialect turkle beside turtle is the usual modern illustration. Beadle itself is native enough, from Old English bydel, a herald or messenger, from a Germanic officer-word. If the reshaping were real, the hound-name and the spy-name would be the same word from the start. It remains an alternative, not a demonstration.
None of these paths has a decisive early form that names a dog in the donor language. Fifteenth-century English acquired a name for a small hare hound; the most discussed source is a French word for a gaping, noisy mouth, with a Celtic word for small and a variant of beadle still in the notes.
From the field to the officer
The core sense has been stable: a small scenting hound, especially for hare. Figurative uses grow directly from that work. From the mid-sixteenth century a person who sniffs out or hunts down others could be called a beagle: a spy, an informer, a sheriff's officer, a bailiff. Hall's chronicle in 1548 already likens pursuers to good begeles following their prey. Collins still marks the spy sense as archaic, but the metaphor never quite left. The playful legal beagle, a lawyer who tracks a case, depends on the same scenting image, with a later nod to legal eagle.
The verb followed the noun. To hunt with beagles is to beagle; by a thinner extension, some writers have used it for searching in general. The ship's name Beagle, later famous through Darwin's voyage, is the breed name transferred to a vessel, not a separate etymology. What later centuries standardized as a kennel breed is a tightening of the older hunting type, not a new word.
A date firmer than a root
The firmest fact is chronological. By the later fifteenth century English already distinguished the small begle from the larger rache in the hunting pack. The French loudmouth remains the best-known guess, and the later French borrowing of the English breed name is a warning not to reverse that later traffic for the earlier one. The word arrived as a hound's name; the spy, the bailiff, and the courtroom tracker are the scenting sense let loose on people.