Awkward now, handy then
Call a person ungainly and you mean the body does not cooperate: long limbs that swing wide, a gait that looks unfinished, a presence that takes more space than it needs. The same adjective covers objects that are a trial to lift or steer, and shapes that simply look wrong in their setting. A tall man folding himself into a small car, a cello on a crowded bus, a strip of shops that sits badly on a road: all of these can be ungainly. The core idea is lack of ease.
That is a physical judgment today. The older English of the word was not about posture at all. Before it described a clumsy walk, it described something inconvenient, unbecoming, or unfit for the job. The history is a slow slide from handy versus unhandy into graceful versus graceless.
Not the gain you think
The syllable gain inside ungainly has nothing to do with profit, increase, or getting ahead. Those senses came into English from Anglo-French and live on in to gain an advantage and economic gains. The gain here is an obsolete adjective that once meant direct, suitable, handy, or well disposed. Prefix un- and suffix -ly sit on that stem, so the surface analysis is simply not-gainly.
The positive partner gainly still exists, but it is rare. In the sense well-formed and agile it is a late nineteenth-century coinage, first recorded in the 1880s, and it is probably peeled off the negative form rather than the other way around. An earlier gainly meaning ready or prompt, from the 1620s, belongs with the profit-noun, not with this family. For most speakers the negative is the only survivor, which is why the word now looks as if it commented on money.
A Norse word for a straight line
The stem reached Middle English as gein, geyn, or gayn, taken from Old Norse gegn. In Norse the word could describe a path that ran straight, and by an easy step a person or deed that was fitting, serviceable, honest, or kindly. Straightness of route and fitness of conduct sit close together: the right way is the one that does not wander. A helpful neighbour and a short road share the same geometry.
That root family also produced English again and against, German gegen, and Scandinavian forms such as Swedish gen and Danish igen. Proto-Germanic gagin (sometimes cited as gagina) meant toward or against. How that particle arose is not settled. Some linguists have linked it to the verb for going; others treat the deeper origin as unknown. What is clear is a northern loan into English during the period of Norse contact, not a Latin or French import.
Dictionaries do not tell the English side in identical words. Merriam-Webster traces the obsolete adjective gain through Old English gēn as well as Middle English, then back to Norse gegn. Etymonline, Wiktionary, and Dictionary.com treat the Middle English forms as borrowed straight from Old Norse. Either account still puts gegn at the source. Old English did have a related negative ungænge, meaning useless or vain, and early fourteenth-century English had a verb ungeinen, to harm or fail to help. Those look like cousins on the same Germanic branch, not the immediate parents of the modern spelling.
Adverb first, clumsy later
Sources do not date the word as a single event. The adverb ungeinliche is already Middle English, with some dictionaries placing it in the late twelfth century. It was built on ungein, inconvenient, disagreeable, or troublesome: the opposite of handy and well disposed. By around 1400 an adjectival use meaning unfit or improper is recorded. The sense that now dominates, awkward or clumsy in body, is early seventeenth century. The Oxford English Dictionary’s first evidence for that adjective is 1611, in the work of the lexicographer Randle Cotgrave. Etymonline gives the clumsy sense as the 1610s and notes the earlier unfit sense. American Heritage analyses the modern adjective as un- plus obsolete gainly, itself from Middle English gainli and Norse gegn in the meaning direct.
So the origin of the word ungainly is not a sudden label for gawky teenagers. It is a native English negative of a Norse fitness-word, first used of things that were inconvenient or unbecoming, and only later narrowed onto gait, bulk, and handling. Older dictionaries still list an obsolete sense unsuitable or unprofitable, which keeps the moral and practical coloring of the Middle English period. Collins still notes a rare sense crude or coarse, at the edge of the same field.
Whether the modern adjective is un- plus obsolete gainly, or ungein plus -ly, or a conversion of the adverb, is a matter of analysis more than a fight about the parts. All three descriptions point at the same pieces: a negative prefix, a Norse stem for straight and fitting, and an English adverbial ending.
What dropped away
Old Norse itself had ógegn, glossed as ungentle or, in older bilingual notes, as the very quality English later called ungainly. Dialectal Norwegian Nynorsk ugjegnleg still means stubborn or troublesome. Northern English and Scots kept a positive gain or gane for a short, direct road, the ganest way. Those survivals show the original map: against, toward, straight, therefore convenient. Once gain as handy faded from standard English, ungainly was left looking like a comment on profit, which it never was.
The useful takeaway is chronological. If you want the oldest English layer, look at the Middle English adverb of inconvenience. If you want the familiar insult of clumsy movement, you are in the 1610s. And if you meet gainly in a modern sentence meaning graceful, you are probably looking at a back-formation that needed ungainly in order to exist.