A material that does not quite close
In present-day English, porous names a solid that is full of tiny openings. Water soaks into unglazed clay. Air and liquid move through a sponge. Rock may hold oil or groundwater in the empty spaces of its fabric. The adjective also travels past matter. A border that people cross with little obstruction is porous. A defense that keeps leaking goals is porous. A rule that outsiders can influence is porous. In each case the picture is the same: what looks like a wall is actually a set of passages.
The origin of the word porous belongs to that picture of a passage, not to later laboratory talk of permeability as a measured property.
Borrowed in the later 1300s
English already had the adjective in the later fourteenth century. First-use notes cluster in that century: some accounts give the broader fourteenth century, others a window from about 1350 to 1400, others the late 1300s. It is a borrowing. It does not continue an Old English formation of the same shape.
The last hop into English has more than one plausible donor, and the sources do not force a single winner. Fourteenth-century Old French had poros, later French poreux. Medieval Latin had porosus, an adjective built on Latin porus, an opening. Middle English also used porose, and some dictionaries treat porous as a variant of that form. Other accounts route the Middle English word through Old French poreux and poros, then Medieval Latin porosus, then the Latin noun. A further possibility is that English attached its own -ous to Latin porus. All of those paths end at the same Latin noun. What is not in doubt is the period and the direction: learned and semi-learned Latin, with French in the neighborhood, into Middle English.
The noun pore arrives in the same late-fourteenth-century stretch, from Old French pore and Latin porus, for a minute opening in skin, wood, stone, bone, or earth. Adjective and noun came in as a pair. A close twin, porose, is on record around 1400 and still turns up in some biological writing; ordinary English kept porous.
From a Greek way across
Latin porus is itself a loan from Greek poros. In Greek the noun was wider than a hole in the skin. It meant a means of passage: a way, an opening, a ford, a strait, a journey, a resource, a way out of a difficulty. The anatomical pore is one narrowed use of that older idea of a crossing-place.
The Greek noun is the o-grade of the verb that means to pierce or run through. Reconstructors take both back to a Proto-Indo-European form for a passage or journey, from a verbal root meaning to lead or to pass over. That reconstruction is shared across standard etymological handbooks; it is not a story invented to tidy this one English adjective.
Latin morphology is equally plain. Porosus is porus plus the suffix that means full of. English speakers later read the same shape as pore plus -ous. That surface analysis is fair, even though the suffix had already been attached before the word settled in English. The adjective does not hide a folk etymology. It says, quite literally, full of openings.
Holes first, leaky barriers later
The earliest English sense is physical: full of pores, able to let something through because of small perforations. That remains the core in crafts and in science. Potters glaze a vessel so the clay body will not drink what is poured into it. Bone that has lost mineral can be called porous. In geology and materials science the word names a property that can be measured: how much of a solid is empty space that a fluid can occupy or cross.
From the 1640s the adjective is also used in a transferred way. Once a perforated solid was a familiar image, it was a short step to any barrier that fails to keep things out. Modern prose leans on that extension. Borders, defenses, and regulations are porous when they do not hold. Related senses include openness to outside influence and the mere capacity to be penetrated. The metaphor does not cancel the material meaning. It sits beside it.
What did not happen is a large drift away from the idea of passage. Some borrowings pick up a moral or social sense and drop the physical one. This one did not. A sponge and a weakly guarded frontier are still, in this vocabulary, things you can pass through.
Cousins that never went through the skin
Because the older root is about going across, English holds several native kin that never passed through Latin porus. Fare, ferry, and ford belong to the same family. So, at a greater distance, do Latin porta, a gate, and portus, a harbor, which is why port, portal, and opportunity sit in the same neighborhood. Greek emporos, a traveler or merchant, is the source of emporium. French poreux, Italian and Spanish poroso, and German porös are the obvious Romance and later European adjectives built on the same Latin stock.
Those kin are useful because they show what the Greek noun meant before anatomy claimed a share of it. A ford is a passage through water. A pore is a passage through tissue. Porous simply says that the passages are many.
One spelling trap is worth marking. The English verb to pore, as in to pore over a page, is not part of this history. That verb is older in English, from the early thirteenth century, and its source is uncertain. The similar look is coincidence, not shared descent.
The concrete residue is a date and an image. By the late fourteenth century English had borrowed an adjective that meant full of openings. The openings themselves were named from a Greek word for a way across. Four centuries later the same adjective could describe a border as easily as a sponge, because both, in this vocabulary, are things with a way through.