Not a globe at first
In present-day English, world covers a great deal: the planet and the life on it, the universe as a whole, human society, a trade or pastime, secular life as opposed to the life to come, even a large amount. The origin of the word world does not start with maps or astronomy. It starts with a span of time lived by people.
That older picture is still visible if you listen for it. To bring a child into the world is to start a life, not to land on a continent. A man of the world is someone at home in public affairs. Worldly still means attached to this life rather than to heaven. Those uses are not late metaphors piled onto a planetary noun. They are closer to what the word first did.
A native compound, not a loan
English inherited the noun. It did not borrow it from Latin mundus or French monde. Old English already had weorold, with a swarm of spellings beside it: weoruld, woruld, worold, world, and others. The earliest English evidence belongs to the Old English period, before 1150. By the ninth century the same word could already name the earth as a created place, but that was an extension, not the core.
The form is Germanic through and through. Comparative reconstruction gives Proto-West Germanic weraldi, from Proto-Germanic weraldiz. That is a compound, and the two halves are still recognizable once they are named.
Man plus age
The first element is Proto-Germanic weraz, man. Old English kept it as wer. Everyday English later dropped that word in favor of man, so the link is easy to miss. It survives in werewolf, literally man-wolf, and in the old legal term wergeld, the man-price paid for a killing. The same Indo-European root yields Latin vir, the source of virile and virtue, and Sanskrit vira. Celtic languages keep related forms as well, among them Old Irish fer.
The second element is aldiz, age or lifetime, the same stock as English old. The Germanic adjective behind old began as a past-participle sense, grown up or adult, and is usually traced to an Indo-European root meaning to grow or nourish. Put the two pieces together and the compound means, quite plainly, the age of man: a human lifetime, an era of humankind, the time in which people live.
That analysis is the standard one. The compound itself is not attested in writing from the Proto-Germanic period; it is inferred from the daughter languages. The inference is strong because the same structure turns up across the family with matching sense.
What Old English actually named
In Old English the noun was feminine. Its range was wide, but the physical planet was not the center of it. Speakers used it for human existence in this life, for the cares and pleasures of the present state, for mankind as a group, for a long stretch of time, and for a person’s own lifetime. The present world could be set against the next. The formula world without end, which translates the Latin liturgical phrase for ages of ages, keeps the time sense almost intact.
Church English had another, more spatial word for the inhabited earth: middangeard, the middle enclosure, kin to Old Norse Midgard. In the Old English gospels that was often the ordinary choice for the physical world. Weorold competed with it rather than replacing a learned loan. Gothic, rendering Greek kosmos, sometimes used seed of man. Old Norse often preferred heimr, an abode. The English word that won was the one built on lifetime, not on a yard or a home.
The two main modern senses were already in play before the Norman Conquest. A late ninth-century translation of Gregory’s Pastoral Care uses the word for earthly, human affairs. Verse associated with Cynewulf uses it of the created world that God made. One word could hold a lifespan, a society, and a place.
How the meaning spread
By about 1200 the noun had stretched to the known world, then to creation and the universe in the broadest sense, and to any state or sphere of existence. Emphatic talk such as how in the world appears around the same time. In the middle of the fourteenth century, theological writing often used it for that part of humanity given over to secular pleasure. For all the world, meaning in every particular, is from the same century. Late in the 1300s a world of could mean a great quantity.
Geography came later. From the 1550s the word could pick out a portion of the globe, as in New World. Around 1600 it could mean a body of people bound by faith or heritage. By 1713 it could name a planetary body imagined as peopled. Later phrases kept multiplying: world-wearied by the 1590s, world power by 1900. Out of this world once meant dead; the slang sense of marvelous is from 1928.
None of this required a new etymology. Speakers kept the old compound and pointed it at larger and larger frames: a life, a people, a planet, a cosmos.
A Germanic family, and a useful parallel
Cognates make the history visible. Dutch has wereld, German Welt, Old Norse veröld, Old Saxon werold, Old Frisian warld, Swedish värld, Danish verden. Scots kept warld. The vowels and consonants shift, but the man-plus-age structure remains.
The pairing of age and world is not unique to Germanic thought, even if this particular compound is. Latin saeculum can mean a generation, an age, or the present world. Greek aion has a similar spread. Those are independent words, not sources of English world, but they show why a lifetime-word can become a universe-word. If the world is the time in which humans live, it is a short step to the place, the society, and the created order that fill that time.
Other Indo-European languages built their ordinary world-words from different pictures: a foundation in some Celtic and Slavic forms, something under the sun in Lithuanian. English stayed with the age of man.
The half of the word that English forgot
World looks like a simple noun for the earth, but it is a fossil compound. The second half is old, still a living English word. The first half is wer, a word English otherwise almost lost. When you say world you are still saying man-age, even though only werewolf keeps that first element in plain sight. The planet sense is real, and old, but it is a later pointing of a word that began as a lifetime.