подмышка

под- (pod-, “under”) +‎ мы́шка (mýška, “"mouse", meaning "muscle" (see мы́шца (mýšca) for more)”)

• IPA^(key): [pɐdˈmɨʂkə]

подмы́шка • (podmýška) f inan (genitive подмы́шки, nominative plural подмы́шки, genitive plural подмы́шек, relational adjective подмы́шечный)

1. (anatomy) armpit, underarm

   Synonym: подмышечная я́мка (podmyšečnaja jámka)

   Я пря́чу кни́гу под мы́шкой.

   Ja prjáču knígu pod mýškoj.

   I hide a book in my armpit.

• When preceded by the prefix под (pod, “under”), the first part of the word is omitted:

  Я несу́ портфе́ль под мы́шкой. ― Ja nesú portfélʹ pod mýškoj. ― I am carrying a briefcase in my armpit.

  Я положу́ э́тот мешо́к себе́ под мы́шку ― Ja položú étot mešók sebé pod mýšku ― I will put this bag in my armpit.

This seems to be a historically, rather than grammatically, defined feature, since originally the main form was simply мышка. This historical noun is still present in other Slavic languages, cf. Macedonian мишка (miška).