What speakers mean by it now
Call a person mousy and you are almost never paying a compliment. In present-day English the adjective gathers several likenesses to the animal at once: a timid or retiring manner, a quiet or stealthy way of moving, and a dull greyish-brown colour, above all of hair. Dictionaries still list a more literal reading as well, a place or thing full of mice, the same kind of sense that dusty or weedy carry. A spelling mousey is common, and speakers form mousier, mousiest, and the noun mousiness without strain.
None of those uses required a loanword. The origin of the word mousy is a home-grown English adjective of the early nineteenth century: the inherited noun mouse plus the native suffix that turns a thing into a quality.
Built in English, not borrowed as a unit
The suffix is the ordinary adjective-maker from Old English -ig, later spelled -y. With verbs it has been productive since the thirteenth century (drowsy, clingy); with nouns and other bases it marks “having the character of” or “abounding in.” Animal names of one syllable take it easily. On that morphological reading, a mousy pantry is simply a pantry with mice in it, just as a ratty barn is a barn with rats. That concrete sense is the least fashionable today, but it is the one the ending itself predicts.
The social and colour readings are later overlays on the same formation. Independent dictionaries place the first written evidence in the early 1800s. Several give 1812 as the first known use, with quietness or stealth as the sense in view; others report a first-recorded window of 1805–15, the same decade as a range rather than a single year. The colour sense, greyish brown like a house mouse’s coat, is recorded from 1853, roughly forty years later. The personality reading — meek, unassertive, easy to overlook — grew out of the quiet sense and is now the one most likely to sting.
The spelling with -ey is a variant, not a separate word. Both forms keep the diphthong of mouse; some speakers voice the s, some do not.
The noun was already a character before the adjective existed
Mouse itself is native. Middle English had mous, from Old English mus, a word for the small rodent that also, in Old English, named a muscle of the arm. That double duty is not a joke later writers invented. A muscle under the skin can look like a small creature slipping about, and the same image appears in Greek, where mys is both mouse and muscle and supplies the medical combining form myo-. German Maus still covers both meanings in some older anatomical use. English later borrowed Latin musculus, a diminutive of mus, as muscle, so the scientific word and the homely adjective sit on two branches of one idea.
The rodent sense is far older than any of that. Old English mus continues Proto-Germanic *mūs (handbooks also write *mus), with close kin in Old Norse mus, Dutch muis, and German Maus. Behind the Germanic form stands an Indo-European name for the animal that survived in several families: Latin mus, Greek mys, Sanskrit mus or mūṣ, Old Church Slavonic mysu (Russian myšʹ), and Persian muš. Reconstructions of the remote ancestor differ in detail — some handbooks print *mus-, others a laryngeal shape *múHs — but they are aiming at the same inherited word, not at a later borrowing into English.
The irregular plural mice, from Old English mys, shows i-mutation: a front vowel in the ending pulled the stem vowel forward. That change belongs to the noun’s early history and has nothing to do with the later adjective, which inflects in the regular way.
By the late fourteenth century, well before anyone wrote mousy, English already treated the mouse as a type of the timid or the weak. Early modern English could set mouse against man in proverbial pairings attested from the 1620s. The nineteenth-century adjective did not invent the metaphor. It packaged an old comparison as an everyday describing word.
How the senses drifted
The path of meaning is short and fairly clear, which is not always true of animal adjectives. First comes resemblance in behaviour: quiet, stealthy, easily startled. That matches how people watch a mouse move along a skirting board. From quietness it is a small step to character: not merely silent, but shrinking, ineffectual, unwilling to claim space. Colour arrives later as a second, independent likeness to the fur, and it settled especially on hair. Mousy brown is now a fixed collocation; it names a shade that is neither strongly blonde nor clearly dark, and it often carries a hint of drabness even when no personality is being judged.
The infestation sense never disappeared from the dictionaries, but it lost ground in ordinary prose. When a house is full of mice, speakers more often say so in a clause than by calling the house mousy. The adjective’s energy went into people and palettes.
What the word did not do is follow the computer mouse. That device sense of the noun dates from the 1960s (shape-names for mouse-like tools are older in nautical English, from about 1750). The adjective’s history was already set in the 1800s, and mousy still points at the animal, not at a pointing device.
Sources disagree mainly on how tightly to date the first adjective, not on the anatomy of the word. A single year, 1812, is the figure most often repeated; a span covering the decade around 1810 is the more cautious report. Neither account disputes the parts: mouse plus -y. There is no serious rival etymology and no need for a folk story.
A leftover from a much older name
The useful takeaway is not that English once noticed mice were quiet — that is obvious — but that mousy is a late, transparent badge on a noun that Indo-European languages have been sharing for millennia. Latin mus, Greek mys, and English mouse are not coincidental lookalikes; they are inheritances of one old animal name. The English adjective is young. The creature-word underneath it is not. And because that noun also named a muscle, mousy and muscular are distant relatives: one describes a timid manner or a grey-brown coat, the other a little mouse of flesh. The contrast is accidental in modern usage, but it is sitting in the history if you know where to look.