mūs-

PIE word for mouse.‌​‌​‍​‍​‌​‍​‌​‍​‌​‌​‍​‍​‍​‌​‍​‍​‍​‌​‌​‍​‍​‌​‌​‍​‍​‌​‍​‍​‌​‍​‍​‍ Gives Latin mūs, Greek mŷs, English "mouse", Sanskrit mūṣ.

Discussion

*mūs- is the Proto-Indo-European word for "mouse," one of the most remarkably stable animal terms in the language family.‌​‌​‍​‍​‌​‍​‌​‍​‌​‌​‍​‍​‍​‌​‍​‍​‍​‌​‌​‍​‍​‌​‌​‍​‍​‌​‍​‍​‌​‍​‍​‍ The near-identical form across branches suggests the word was already well established in the proto-language.

English "mouse" descends directly through Proto-Germanic *mūs, Old English mūs. The irregular plural "mice" (from earlier *mūsiz) preserves an ancient i-mutation pattern. German Maus, Dutch muis, and Old Norse mús continue the same form.

Latin mūs "mouse" is virtually identical to the PIE original, and from it English borrows "murine" (relating to mice) and "muscle" — from mūsculus, literally "little mouse," because the Romans thought a flexing muscle resembled a mouse moving under the skin.

Greek μῦς (mûs) "mouse, muscle" shows the same semantic extension from rodent to muscle, confirming this metaphor predates the individual branches. Sanskrit mūṣ- "mouse" and Avestan mūš- continue the root in the Indo-Iranian branch.

Old Church Slavonic myšĭ and Lithuanian mušìs demonstrate the Balto-Slavic reflexes. The word has even been borrowed into non-IE languages. The extraordinary phonological conservatism of this word across five millennia makes it a textbook example of lexical stability.

Notes

Also source of "muscle" (Latin musculus, "little mouse" — from muscle movement under skin)