By surface analysis, миш (miš) (obsolete) + -ок (-ok).
мишо́к • (mišók) m (feminine ми́шка)
- male mouse
- big mouse
- “мишок”, in Речник на българския език [Dictionary of the Bulgarian Language] (in Bulgarian), Sofia: Bulgarian Academy of Sciences, 2014
- “мишок”, in Речник на българския език [Dictionary of the Bulgarian Language] (in Bulgarian), Chitanka, 2010