[cesis]

I often have trouble with claimed cognate words without any references or analysis. This very often applies also to Wiktionary.

E.g. even the given example - I would find it believable that "muscle" cognates with Latvian "miesa", Russian "мяса" and English "meat", but "mouse" seems sketchy.

[kian]

I had a teacher in high school, specializing in ancient latin and greek, who told me about the musculus --> little mice connection. He was also a very strong Austrian -- to demonstrate the reason, he pulled up his shirt sleeve, made a classic bicep curl motion, and rotated his fist rapidly from inside to outside facing. It was pretty shocking to see the 'little mice' running under the 'covers' of the skin of his arm -- since then, I've never had trouble believing this particular hypothesis.

[arnsholt]

To add on this, musculus is not only an entirely regular diminutive derivation of mouse (mus + -culus) it's even the same pattern as is used for testicle: it's a small witness (testis, see also latinate English words like testify) of manhood.

[dredmorbius]

TIL:

testis (n.) (plural testes), 1704, from Latin testis "testicle," usually regarded as a special application of testis "witness" (see testament), presumably because it "bears witness to male virility" [Barnhart]. Stories that trace the use of the Latin word to some supposed swearing-in ceremony are modern and groundless.

https://www.etymonline.com/word/testis

[johtso]

This is my go to site for etymology https://www.etymonline.com/word/muscle

Has a good description of this derivation.

[Ericson2314]

Yeah I usually cross-reference etymonline and wiktionary, and I can't recall anything too suspicious.