Water from thaw, not from a tap
In current English, meltwater is the liquid released when snow or ice turns to water. Hydrologists and physical geographers use it for runoff from a snowpack, a glacier, or an ice sheet: the stream at a glacier snout, the spring pulse in a mountain valley, the film that drains across ice in summer. Major dictionaries keep that definition short and almost identical. They do not stretch it to molten metal, to a cheese sandwich, or to the figurative softening of a heart, even though those senses live in the verb melt.
The noun is both mass and count. Writers speak of meltwater as a substance and of meltwaters from separate glaciers. The core meaning has not wandered. What has grown is the technical company it keeps: ablation, outwash, runoff, and, in climate research, sudden pulses of freshwater into the ocean.
Built in English, not imported
The origin of the word meltwater is a native join, not a loan. English set melt in front of water and treated the pair as one noun. Wiktionary describes the formation in those two parts. Collins, Dictionary.com, and the American Heritage Dictionary all gloss the same compound.
The date of first print is less tidy. Merriam-Webster’s first known use is 1923. Collins and Dictionary.com give a window from 1930 to 1935. A gap of that size is common when a phrase has already been written with a space or a hyphen and lexicographers later lock the closed spelling as the headword. English glacier writing had needed a name for water released by ice long before the 1920s; the fused noun is a twentieth-century label for an older fact.
The English word is not a borrowing from German Schmelzwasser or Dutch smeltwater, though those are close kin. Each language built the same picture from its own verb of melting and its own word for water.
Two old Germanic halves
Melt is the older and heavier half. Old English had meltan, an intransitive strong verb meaning to become liquid through heat, with past mealt and past participle molten, a form that still survives as an adjective. Beside it stood a causative, West Saxon gemyltan and Anglian gemæltan, to make something liquid. Middle English folded the two into melten. From that merger come modern melt, melted, and the fossil molten.
Behind Old English lies Proto-Germanic meltaną, with a related causative maltijaną. Old Norse and modern Icelandic melta, to digest, is a cousin: digestion pictured as an internal melting. Reconstructors take the family to a Proto-Indo-European root for making something soft or liquid, written meldh- in some handbooks and (s)meld- in others. From that neighborhood come Greek meldein, to melt, Sanskrit mrduh, soft, and Latin mollis, soft. English smelt, the metalworking verb, is a related Germanic form with an extra initial s.
Water needs less unpacking. Old English wæter continues Proto-Germanic watr- or watōr, from the Indo-European root wed-, water or wet. German Wasser, Dutch water, Old Norse vatn, and Gothic wato are the expected siblings. Linguists often contrast this inanimate root with another Indo-European water word, ap-, which named water as a living force and survives in names such as Punjab. Meltwater uses the ordinary, inanimate noun.
The morphology is transparent. There is no prefix, no hidden suffix, and no folk story to unpick. Melt here is the verb used attributively: water that results from melting. English already had ice-water by 1722, but that older compound splits between water made from melted ice and water merely cooled with ice. Meltwater keeps only the first of those jobs.
A sense that stayed put
Because the compound is so clear, it did not have to change meaning in order to spread. Its history is one of register, not of sense-shift. In glaciology it became a technical staple. Water on the ice, inside the ice, and under the ice received the labels supraglacial, englacial, and subglacial. Streams of it build outwash plains; sudden releases become outburst floods. Paleoclimate research speaks of meltwater pulses, large freshwater injections at the end of ice ages. None of those uses redefines the noun. They locate it.
Everyday English still has snowmelt and runoff for overlapping facts. Meltwater is the form that sounds like a specimen name: the water as a product of thaw, especially from glacier ice. Cambridge marks it as geography specialist vocabulary, which matches how it is used: not rare, but rarely casual.
A German mirror
If you want a check on the English formation, look at German Schmelzwasser, Dutch smeltwater, or Swedish smältvatten. They are not loans from English, and there is no solid evidence that English calqued them. They are parallel compounds. That parallelism is the useful takeaway. Once a Germanic language has a verb for melting and a noun for water, the join is almost inevitable. English happened to write it as one word in the early twentieth century, and dictionaries still argue by a decade about the year. The argument is about spelling and first citation, not about what the word means.