Today business names a firm, a trade, a field of study, and the private affairs that are none of anyone else's. People do business, go into business, and mind their own business. The commercial sense is so dominant that the word's older life as a plain description of being occupied is easy to miss.
A native compound, not a loan
The origin of the word business is homegrown. It was not borrowed from French or Latin. Speakers of Old English built it from the adjective bisig, later busy, and the abstract suffix -nes, the ancestor of modern -ness. The earliest surviving form is Northumbrian bisignes, recorded before 1150. The Oxford English Dictionary treats that Old English evidence as the first use. Merriam-Webster, working from Middle English bisynesse, dates first known use to the fourteenth century. The disagreement is about which stage of the same native word counts as the starting point, not about a rival source language.
Middle English spellings wander: bisines, bisynesse, besynes, busynesse, and many more. The u in the modern spelling reflects West Midland and Southern pronunciations of busy. The spoken vowel in the first syllable comes from East Midland speech, which is why English writes a u and says a short i.
From three syllables to two
For a long time the noun was a transparent state-word, busy plus -ness, with three beats. From the sixteenth century, spellings that drop the middle vowel begin to appear. Pronouncing authorities of the early seventeenth century already treat the two-syllable form as normal. The modern BIZ-niss pronunciation is the result of that syncope. Once the word no longer sounded like busy-ness, it was free to drift away from the adjective that created it.
Samuel Johnson still listed busiless for a person at leisure, without occupation. That formation only makes sense while business still felt like the state of being busy. After the extra syllable collapsed, the spelling kept a fossil of the old morphology that the spoken word no longer matches.
West Germanic relatives
Busy itself goes back to Proto-West Germanic *bisīg, reconstructed with the sense diligent, zealous, occupied. Close cousins survive outside English: Dutch bezig, Low German besig, Saterland Frisian biesich, and Afrikaans besig. The family is West Germanic rather than pan-Germanic. A matching North Germanic adjective is not securely attested, and a deeper Indo-European root is not agreed on. One cautious proposal links *bisīg to a verb *bisōn, used of rushing or whirring about, plus an adjective-forming suffix. That remains a guess, and it should not be presented as settled history.
Old English also had the verb bisgian, to occupy, employ, or trouble someone. Dutch bezigen, to use, is a related formation. The English noun therefore sits on a small cluster of West Germanic words for being taken up with activity, not on a Latin commercial term such as negotium. The later mercantile meaning is an English development, not an imported label for trade.
Anxiety, occupation, then trade
The oldest English senses are care, anxiety, and occupation. Bisig itself could mean careful and anxious as well as diligent. That unease later drained out of the noun. What remained in Middle English was the condition of being much occupied, and then, by the late fourteenth century, a person's work, livelihood, or appointed duty.
From there the word widened. By the 1590s it could mean whatever one was about at the moment. In the early seventeenth century, to mind one's own business already meant to attend to one's affairs and leave other people's alone. A now-lost seventeenth-century use treated it as a euphemism for sexual intercourse, another instance of private activity dressed as occupation. Early Modern English also used related busy-forms in a bad sense for prying or meddling, a tone still audible in busybody.
The fully commercial sense — trade, mercantile dealings, buying and selling taken as a sphere of life — is later. Historical dictionaries usually date that meaning to the early eighteenth century, around 1727, on the idea that commerce is simply the set of matters that occupy one's time and attention. Compounds and idioms then pile up: business letter in the 1760s, business card around 1840, to mean business in the 1850s, business as usual in the 1860s, and American English business end for the working part of a tool by the 1870s. The word also specialized as a label for a firm, for volume of trade, for management as a subject of study, and for the practical agenda of a meeting or a stage.
Why English needed a new abstract noun
Once business had become the name of work and commerce, speakers needed a fresh word for mere activity. That slot was filled by busyness, a transparent rebuild of busy plus -ness, with the three syllables restored. The Oxford English Dictionary records the new spelling from the early nineteenth century; other accounts point to Thoreau in 1849 as an early clear use. Either way, the split is recent. For several hundred years the two ideas lived inside one word.
The takeaway is audible as well as semantic. Dutch still uses bezig for occupied, and English still writes the u of busy inside business, but the spoken English noun has been a two-syllable commercial word for about four centuries. The spelling is a fossil of diligence; the meaning is a later bargain.