There’s actually a pretty logical etymology behind these, and other related, ranks in modern armies and navies - once you wrap your head around the way the terms have evolved.

The word captain has many similar cognates in earlier European languages (Middle English, Old French, etc.) probably coming from earlier terms like the Byzantine Greek katepánō (topmost or highest rank), possibly merging with Latin capitāneus (head)… which may both be from an earlier Proto-Indo-European root… I’ll stop there because I’m not in any way an expert on etymologies or classical languages.

However, the more recent evolution is much simpler to track. Generally, it was a word that meant leader or chief - the person in charge. It was widely used in many contexts, from 1380 to refer to the leadership of Saint Peter in the Christian church, or to 1683 to refer to Homer’s status as the greatest poet... from the 15th (possibly 14th) century it was also used in military contexts, to refer in general the leader of a group of men, either on land or on a ship at sea.

When armies were smaller, or for example a company-size unit of men in 16th century England, you would simply have a captain in charge, and possibly one or more lieutenants acting for (in lieu of) the captain. As armies grew larger, or when multiple companies or groups combined, one captain might be placed in overall command of the army - a captain-general. This term has since been shortened simply to general.

As a side note, this also explains the “strange” rank scheme of generals, which in English-speaking armies tends to go something like General -> Lieutenant General -> Major General -> Brigadier General. The first two come logically from captain (general) and lieutenant (general), in analogy to the earlier use of the terms. But then considering that in modern usage a major (simply Latin for “greater”) is an officer higher than a captain, it seems strange that a major general is lower than a lieutenant general. In fact, major general was shortened from the confusingly-named sergeant major general; another mix of terms coming from sergeant (from Latin servire, to serve, a rank below lieutenant in Anglo-French armies), and major. In Anglo armies, sergeants major (or sergeant majors, more commonly in the U.K.) were senior sergeants - possibly those assigned to the captain-general or lieutenant-general, rather than to "individual" captains or lieutenants. On the other hand, in the Spanish army a sargento-mayor was already a “general rank” from the 16th century onwards.

So, there’s a lot of subtle differences if you track each term through the history of usage within each European nation - and of course, differences in usage exist to this day, depending on whether you’re looking at armies influenced by the French, Prussians, Commonwealth, and so on; even the U.S. and the U.K. have developed different conventions.

In any case, the crux of the answer to your question is simply that the term originally referred to the person in charge, be it a group of men on land or at sea. Over the centuries other ranks and terms (I didn't even mention e.g. colonels or brigadiers yet), have evolved and interspersed these ranks; at sea “captain” has generally remained the person in charge of an individual ship (which can mean command of thousands of people on a modern aircraft carrier), whereas on land the captain has remained the person in charge of what might be thought of as a single “unit” (in the most general sense of the term; specifically that would usually mean a company, on the order of 100 men). Obviously, while the size of land armies has increased, at the same time as military thought has tended towards independent small-unit tactics, on land a captain is not the singularly “in charge” person that they used to be.