The correct spelling of the word is "viscosity," which is pronounced /vɪˈskɒsəti/. The word refers to the resistance of a fluid to flow. The spelling "muscosity" appears to be a common misspelling or a typo. It may be confused with the word "mucosity," which has a similar sound but refers to the quality of being mucous or slimy. It is important to use correct spelling when communicating to avoid confusion and misunderstanding.
Mossiness.
Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.