Posture, office, and a stretch of time
Sitting does several jobs in modern English. As a verb form it is the present participle and gerund of sit: a person can be sitting at a desk, sitting for a portrait, or sitting on a board. As a noun it names a spell spent in that posture, a meal service, an appointment with an artist, a court or parliamentary session, or the stretch of time when a bird incubates eggs. As an adjective it can mean simply in a seated position, but it also marks an incumbent, as in a sitting judge or a sitting member of Parliament. The everyday picture is rest on the buttocks or hindquarters. English has long used the same stem for staying put, holding an official seat, and being in session.
Inherited, not borrowed
The verb behind the form is native. Modern sit comes through Middle English sitten from Old English sittan, a class V strong verb whose older past was sæt and whose past participle was seten. Already in Old English the verb covered more than a chair. It could mean to occupy a seat, to sit down, to remain or continue, to settle or encamp, to lie in wait, and even to besiege. That range matters, because later nouns and adjectives grew as much from being in place as from physical posture.
The noun sitting is formed inside English from that verb plus the suffix -ing. Dictionaries do not all date the noun the same way. The Oxford English Dictionary and Merriam-Webster place it in Old English, before the twelfth century; an early English psalter already uses sittinge to render Latin sessionem. Etymonline instead records the verbal noun from around 1200 in the sense of dwelling, and from the mid-fourteenth century as the act of sitting down. The disagreement is about how early the derived noun is attested, not about where the verb came from. Parallel formations appear in related Germanic languages, among them Dutch zitting and German Sitzung, both used for a sitting or a session.
Germanic cousins of sittan include Old Saxon sittian, Old Frisian sitta, Old High German sizzan (modern German sitzen), Dutch zitten, Old Norse sitja (Swedish sitta, Icelandic sitja), and Gothic sitan. Reconstructors usually write Proto-Germanic sitjaną; some older handbooks use a form like setjan. Either reconstruction points the same way: English did not import sit from French or Latin. It inherited the verb from West Germanic. The origin of the word sitting, then, is a native derivation on top of that inherited verb.
A root shared across Europe
Behind the Germanic verb lies the Proto-Indo-European root sed-, meaning to sit. One common reconstruction of the present that yielded the Germanic verb is sédyeti. That same root feeds a large family of words for sitting, seating, and staying put. Latin sedeō, I sit, is the source of English session, sedentary, sediment, preside, and reside. Greek hézomai, to sit, and hédra, seat, belong in the same cluster. Sanskrit sīdati, sits, Old Irish saidid, and Old Church Slavonic sěděti (Russian sidet’) show how widely the root spread.
English itself keeps several descendants of sed- that look different from sit. Set and settle are related Germanic verbs. Seat belongs here too. Nest is a more distant member of the family, historically a sitting place. The Latin loan session is almost a semantic twin of sitting when the topic is a court or a parliament: both name a meeting because the people involved occupy seats. That is not a claim that sitting was borrowed from session. Two branches of the same old root, one native and one Latin, ended up naming the same kind of gathering.
From remaining in place to holding office
Because Old English sittan already meant to remain and to occupy a seat of authority, later English could stretch sitting without changing the stem. Occupying a judicial seat is an old sense of sit; the phrase sit in judgement rests on that idea. Having a seat in a legislative assembly is recorded from the late fourteenth century; of the assembly itself, to hold a session, from the early sixteenth. The noun sitting as a court session appears in the mid-fourteenth century, and as a meeting of a body for business around 1400.
Other extensions stay closer to the body. Posing for a portrait is recorded for the verb by the 1530s; the noun as an interval spent sitting for some purpose, especially a portrait, is dated to 1706. Meal sittings, first and second service, are the same time-slice idea. Birds sitting on eggs, and a sitting as a clutch, keep the physical image of staying put. Later spiritualist usage treated a sitting as a séance, another scheduled interval of remaining in place.
The adjective has its own side paths. Present-participle sitting in the sense that sits is recorded from the early seventeenth century. An earlier, now obsolete, sense of sitting as fitting, said of clothes, belongs to the thirteenth century and matches the verb sense in which a garment sits well. That sits in or holds an office is dated to 1706, the ancestor of today’s sitting president. Idioms arrived later: sitting pretty by 1916, and sitting duck as wartime slang for an easy target by 1944, after a literal use for a bird at rest from the nineteenth century.
The suffix is ordinary; the verb is old
What is easy to miss is how plain the morphology is. Sitting is not an exotic borrowing with a hidden stem. It is sit plus -ing, doing the same work that -ing does on hundreds of English verbs. The history worth keeping is older: a West Germanic strong verb, a Proto-Indo-European root for sitting, and a habit of using being seated as a figure for remaining, ruling, and meeting. When a legislature is in sitting, English is still using the same image that Latin used in sessio, only through a native verb that never left the language.