"Business" looks like it should have something to do with "busy," and that's exactly right — the two words share the same root. The story starts in Old English, where "bisig" meant careful, anxious, occupied, or diligent. Tack on the ending "-ness" (which turns an adjective into a noun) and you get "bisignes," an Old Northumbrian word meaning care, anxiety, or occupation. That evolved into Middle English "bisinesse," which eventually smoothed out into the "business" we write today. The spelling has stayed close to its roots — it's really just "busy" plus "-ness," even though we no longer pronounce it that way. Back in the Middle Ages it was said with three syllables, but by the 1600s people had contracted it down to the two-syllable "biz-ness" we use now, leaving the spelling looking a bit old-fashioned compared to how we actually say it.
As for meaning, the word has drifted quite a bit over the centuries. Its oldest sense — "anxiety" or "care" — is long gone, and even the next meaning, "being busy or occupied," faded out too, eventually taken over by the separate word "busyness." By the late 1300s, "business" had come to mean a person's job or occupation — what they do to earn a living — and soon after, it also meant "a duty one takes on." By the 1590s it could simply mean "what someone is doing right now," and by the early 1700s it had picked up its now-familiar meaning of trade or commercial activity in general — literally the things that occupy your time and attention. Along the way it even picked up a cheeky, temporary meaning: in the 1600s, "business" was sometimes slang for sex. Later centuries added everyday phrases built from the word, like "business card" (1840s), "business as usual" (1865), "mean business" (1856), and the much older "mind your own business," which dates back to the 1620s.
business (n.) Middle English bisinesse , from Old English bisignes (Northumbrian) "care, anxiety, occupation," from bisig "careful, anxious, busy, occupied, diligent" (see busy (adj.)) + -ness . The original sense is obsolete, as is the Middle English sense of "state of being much occupied or engaged" (mid-14c.), the latter replaced by busyness . Johnson's dictionary also has busiless "At leisure; without business; unemployed." The modern two-syllable pronunciation is from 17c. The sense of "a person's work, occupation, that which one does for a livelihood" is recorded late 14c. (in late Old English bisig appears as a noun with the sense "occupation, state of employment"). The sense of "that which is undertaken as a duty" is from late 14c. The meaning "what one is about at the moment" is from 1590s. The sense of "trade, commercial engagements, mercantile pursuits collectively" is attested by 1727, on the notion of "matters which occupy one's time and attention." In 17c. business also could mean "sexual intercourse." Business card is attested from 1840; business letter from 1766. Business end "the practical or effective part" (of something) is American English, by 1874. Phrase business as usual attested from 1865. To mean business "be intent on serious action" is from 1856. To mind (one's) own business "attend to one's affairs and not meddle with those of others" is from 1620s.