Occupied, not idle
In current English, busy mainly marks a person with work in hand, a place full of movement, or a telephone line already taken. A pattern or a layout can be busy when it is packed with detail to the point of distraction. The word also still carries, in some uses, a hint of meddling. That mix is modern in its extras, but the core idea is old: someone or something that will not sit still.
The origin of the word busy is native. It was not taken from French or Latin. It was already an English adjective when everyday speech was still largely West Germanic.
Old English forms and the verb beside them
Old English used bisig and bysig. The senses already covered occupied, diligent, careful, and, in some contexts, anxious. The word could describe a person who was employed in a task and a mind that was not at rest. Middle English kept the same stem under a cloud of spellings: bisy, bysy, busie, besy, and others. The form with u became the written standard later; the spoken vowel did not follow it.
The verb is as old as the adjective. Late Old English bisgian meant to attend to something, to be concerned with it, or to trouble oneself over it. By the later fourteenth century the verb could also mean to keep a person engaged, the sense still heard in to busy oneself with a chore. English did not need a foreign verb for that idea; it derived one from its own adjective.
A small West Germanic family
Across the North Sea the same adjective is easy to recognize. Dutch bezig, Afrikaans besig, Low German besig, West Frisian beuzich, and Saterland Frisian biesich all point to a reconstructed Proto-West Germanic form *bisīg, with a meaning in the range of zealous, diligent, and occupied. Scots has related forms such as bissy and besy. German beschäftigt looks like a cousin in sense only; it is built on a different verb of occupying someone, not on this West Germanic adjective.
The family is therefore real, but it is local. It is a coastal and Low Countries group plus English, not a pan-Germanic word with obvious matches in every sister language. That limited spread is one reason later scholars have argued about what, if anything, lies behind *bisīg.
How far the root can be pushed
Here the reference works disagree, and the disagreement is worth keeping in view. Some etymological dictionaries treat the West Germanic set as isolated. They accept the Dutch and Low German cognates, then stop: they see no secure link to the rest of Germanic or to a named Indo-European root, and they refuse to invent one.
Other accounts go a step further. The Oxford English Dictionary connects the adjective to a cluster of continental verbs that meant to rush about or to bolt, especially of cattle: Middle Dutch bisen, Middle Low German bisen, Old High German bisōn, with later Scandinavian forms likely taken from Low German. On that reading *bisīg would mean something like given to rushing around, an adjective in *-īg on a verb of restless motion. A still looser suggestion treats the whole b-plus-sibilant group as sound-symbolic, a noise of bustle rather than a regular inherited root.
None of those deeper links is proven. The honest picture is a well-attested West Germanic adjective, a clear English descent, and an uncertain prehistory.
Why it is spelled with u and said with i
The clash between the spelling busy and the pronunciation with a short i is not random, and it is not mainly the Great Vowel Shift. Old English bisig shows the original short i. Late West Saxon also has bysig, in which the vowel rounded after the labial consonant b. In Middle English that rounded vowel was written u in the West Midlands and the South, the same spelling habit that left bury looking as it does. The East Midlands, whose speech fed the later standard, kept the unrounded i. Standard English therefore writes a southern and western form and pronounces a midland one. Build shows a similar split of letter and sound.
Early printers, some of them trained in the Low Countries, helped freeze the u-spelling once it was in circulation. The dialect mixture was already in the language; print made one regional look official while another regional sound won the spoken standard. Respelling the word as bizy was proposed more than once. It never stuck.
Senses that arrived and senses that left
Not every early meaning survived. Care and anxiety were part of the Old English and Middle English range; that nervous coloring largely drained away after the medieval period. What remained, and then expanded, was occupation and energy: the person at work, the street that will not empty, the day with too many tasks.
A less flattering sense grew beside that. In late Middle English and early Modern English the word could mean prying, officious, or active in other people’s affairs. The inquisitive shade is already visible in the late fourteenth century. It is still audible in busybody, a compound recorded in the sixteenth century, where the second element makes the interference obvious. The polite core and the nosy edge have lived side by side ever since.
Later extensions are easier to date. In the seventeenth century the adjective could be a discreet way of saying sexually active. In 1884 it was applied to a telephone line already in use, the sense behind a busy signal. In 1903 writers used it of display work that was overdetailed and visually cluttered, the sense still applied to a fabric, a poster, or a screen that will not let the eye rest.
A related noun shows how thoroughly the stem was reused. Business began as the adjective plus the suffix -ness. In Old English that formation could mean anxiety or uneasiness, closer to the older emotional sense of the adjective than to ledgers. The commercial meaning grew in the fifteenth century. By the nineteenth century the abstract state of being occupied had to be rebuilt as busyness, because business no longer did that job.
The word that now fills calendars is the same native adjective that once named care as much as labour. Its nearest living cousin is Dutch bezig. The u in the spelling is a fossil of a rounded southern vowel that standard speech never adopted.