Old English bisignes (Northumbrian) "care, anxiety, occupation," from bisig "careful, anxious, busy, occupied, diligent" (see busy (adj.)) + -ness. Middle English sense of "state of being much occupied or engaged" (mid-14c.) is obsolete, replaced by busyness.
Sense of "a person's work, occupation" is first recorded late 14c. (in late Old English bisig (adj.) appears as a noun with the sense "occupation, state of employment"). Meaning "what one is about at the moment" is from 1590s. Sense of "trade, commercial engagements" is first attested 1727. In 17c. it also could mean "sexual intercourse." Modern two-syllable pronunciation is 17c.
Business card first attested 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 is from 1620s. Johnson's dictionary also has busiless "At leisure; without business; unemployed."