A title for the person at the front
In current English a captain is the person in lawful or agreed charge of a defined group: a company of soldiers, a ship, a sports side, a police or fire unit, or a two-pilot airliner. On land the military grade sits above lieutenant and below major. At sea the same title names a much more senior officer. The institutions differ; the job is still command.
That job is older than any modern rank table. The origin of the word captain is a Latin word for the head, first of the body and then of a group of people.
From a physical head to a chief
Latin caput, genitive capitis, meant the physical head and, by a very ordinary metaphor, the person who stood at the top. Late Latin built an adjective capitaneus, chief or prominent, and then used that adjective as a noun for a leader. The makeup is straightforward: the stem capit- plus an adjectival ending of the -aneus type, the kind that marks belonging or character.
English did not inherit this from Old English. It borrowed it. In the fourteenth century the language took Middle English forms such as capitain and capteyn from Old French capitaine, itself a relatively learned recasting of the Late Latin word. Sources do not agree on the exact first English year. Some dictionaries place the earliest records in the second quarter of the century; others put the arrival toward the end. What they share is the first sense: a leader or chief, someone standing at the head of others, including a military commander in a general way rather than a numbered grade on a promotion list.
The same Indo-European root is already native to Germanic in another shape. Old English heafod, German Haupt, and Gothic haubiþ are cousins of Latin caput. They go back to a reconstructed Proto-Indo-European form often given as *kaput-, head. Sanskrit kaput- belongs to that set as well. Captain is not that Germanic inheritance; head is. The English title arrived later, through French, as a Romance loan sitting on top of a family English already had.
Two French roads from one Latin word
Captain has a near twin that took a different path through French. Old French also had chevetaine, recorded in Anglo-French as chiefteyn, a more popular development of the same Late Latin capitaneus. That form entered English earlier, in the early fourteenth century, and became chieftain.
Historical dictionaries of French and English note a useful sound-change test. If the Latin word had lived on in French from a very early date, ordinary wear would have produced a much more reduced shape. Capitaine looks like a later, more bookish re-borrowing that kept the Latin consonants in view. Chevetaine kept more of the spoken French development and is the source of English chieftain. The two English words are therefore doublets: one Late Latin ancestor, two Old French deliveries, two later careers. Chieftain drifted toward clan or tribal leadership and a somewhat literary register. Captain stayed in the world of organized command.
Other European languages took the French or Latin form rather than growing a native military title from their own words for head. French capitaine, Italian capitano, Spanish capitán, Portuguese capitão, and, by later borrowing from French, German Kapitän, Dutch kapitein, and Swedish kapten all sit on the same line. The pattern is continental: a Late Latin head-man word, routed through French, then copied across Germanic and Slavic military vocabularies.
How English narrowed the job
Once the noun was in English, the language kept adding workplaces for it. By the 1550s it named the officer in command of a warship. A decade later it was settling as the army officer who commanded a company, the grade between lieutenant and major. By the early eighteenth century the naval use had widened to the master of almost any vessel. The verb, to captain, appears around the 1590s: to act as leader, to command.
The sporting sense is much later. A team captain, the player who speaks for the side and is treated as its leader, is recorded from the 1820s. From there the word moved into civilian hierarchies that only resemble a company or a crew: captains of industry in nineteenth-century business language, police and fire captains, a head waiter still sometimes called a captain, the pilot in command on a two-person flight deck. None of these uses invent a new metaphor. They reuse the same person-at-the-head sense in a new institution.
One consequence of that reuse is a rank mismatch that still trips people who meet both services. An army, air-force, or marine captain is a relatively junior commissioned officer. A navy captain is a senior officer, the peer of an army colonel, sitting above commander and below rear admiral. The naval title grew from command of a ship; the army title grew from command of a company. English kept one word for both jobs, so the same label now sits at two different heights on two ladders.
What the older form still shows
Middle English spellings such as capitain, capitaigne, and capteyn keep the French shape in view. The modern spelling captain is a later smoothing, but the first syllable still points at Latin caput. That is the useful residue. English already had head; it borrowed captain when it needed a title with the flavor of French and Latin command. The native word stayed anatomical and general. The loanword became the name of an office.
A clean dating to keep is the split with chieftain. Chieftain is in English by the early 1300s, via the popular French form. Captain follows later in the same century, via the more Latinate French form. Two titles grew from one word for a head, and only one of them ended up on a ship.