A person, a device, a job
In American English, a signaler is anyone or anything that sends a signal. The word can name a person waving flags on a hillside, a railway worker among levers and lamps, a soldier whose trade is communications, or, more quietly, a gadget that emits a beep, a flash, or a radio pulse. British spelling usually doubles the consonant: signaller. Dictionaries also treat the form as a near synonym of signalman, an older occupational title with a longer record on the rails and in the services.
The single-l spelling is the American standard. It is an agent noun, not a borrowed French infinitive, even though French happens to write the same string of letters for a verb meaning to report or to point something out. That coincidence is worth keeping in view until the Latin root is on the table.
Built in English, not imported whole
The origin of the word signaler is English compounding of a familiar kind. The noun is signal plus the agent suffix -er: one who signals, or a thing that does the signaling. A standard American dictionary dates that noun to 1872. That is late. The parent noun signal had already been in English for five centuries, and the verb to signal is recorded from 1805. Notes on the verb list signaled, signaler, and signaling as its immediate family. An older English verb, signalize, had been doing some of the same work since the mid-seventeenth century.
So the agent noun did not have to be fetched from abroad. English already had signal, then made a verb from it, then named the person or mechanism that performed the action. An older occupational compound, signalman, is recorded from the 1730s; railway signalman is attested by 1840. Signaler arrives after those titles, as a more general label that can mean the worker, the device, or simply anyone giving a cue.
The suffix is native English equipment. Old English -ere, continued in Middle English, marks the doer of an action. In this word the morphology is transparent: a verb or noun of action, plus the ordinary ending that turns it into a person or an instrument.
From Latin signum to Old French signal
The payload of the word is signal. English took it in the late fourteenth century from Old French signal, also found as seignal, a term for a seal, imprint, sign, or mark. That French form continues Medieval Latin signale, itself the noun use of Late Latin signalis, an adjective meaning pertaining to a sign. Under that adjective sits Classical Latin signum, a broad word for an identifying mark, a token, a military standard, an omen, or a constellation.
Signum is also the source of English sign, which arrived earlier, in the thirteenth century, through Old French signe. Signal is therefore a cousin of sign, not a child of it inside English. Related Latin descendants include signature, signet, designate, and ensign. Across Romance, the same medieval noun shows up as French signal, Italian segnale, Spanish señal, Portuguese sinal, and Catalan senyal. German Signal and Dutch signaal are later learned or borrowed forms of the same Latin stock.
The deeper ancestry of signum is not settled. One account takes it from a Proto-Indo-European root meaning to follow, as if a sign were originally a standard one follows. Another prefers a root meaning to cut, so that a sign began as something carved or cut out. That second view also notes a comparison with Hebrew and Aramaic words for a slaughtering knife, and the possibility that Latin and those Semitic forms share an unknown third source. None of those reconstructions is secure enough to treat as fact. What is secure is the Latin and French stretch of the path.
How the meaning moved
When signal first appears in English, it can still mean a visible indication in a fairly general way, a sense that later faded. It can also name a divine sign or a device on a banner. By the 1590s the word has tightened toward a conventional, agreed-upon sign: a cue to start, stop, or change course. That is the sense that makes a human signaler possible. If a mark is private or accidental, there is no office of giving it. If it is a code both sides have accepted, there is work for a person who knows the code.
The nineteenth century adds an electrical sense: a modulation of current, later of electromagnetic waves. Radio and then television inherit the word in the twentieth century. As the noun stretches from flags and lamps to current and carrier waves, signaler stretches with it. The railway box and the army set remain the most concrete jobs. A British learner dictionary still glosses the doubled-l form as a person who operates railway signals, or a person trained to send and receive signals in the army or navy. American use keeps those senses and also allows a mechanical or electronic sender.
Military communications made the occupational sense durable. Before radios, the work involved lamps, flags, and heliographs; after radios, the same title covered voice procedure, line, and later data. American signaler and British signaller did not split in meaning, only in spelling. In rail transport, the British form is still often interchangeable with signalman for the person who authorizes movements along a stretch of line.
The French verb signaler, attested in the sixteenth century as a derivative of French signal, took a slightly different turn. It can mean to signal, but it is equally at home meaning to report or to bring something to official notice. English signaler never became that verb. English already had to signal, and later to flag and to report. The English noun stayed on the agent side of the family.
A recent derived compound, virtue signaler, shows the agent suffix still at work. It is a polemical label, not an old sense: a signaler is still someone or something that makes a sign others are meant to read.
The lookalike across the Channel
The useful takeaway is the 1872 date set beside the French lookalike. English signaler is a nineteenth-century agent noun built on a late-medieval borrowing. French signaler is a sixteenth-century verb built on the same Romance noun. They share a Latin ancestor and a spelling. They do not share a part of speech or a date of birth. Anyone tracing the English word back through Old French signal and Latin signum is on firm ground. Anyone treating the English noun as a borrowed French infinitive has followed the letters and missed the grammar.