antepenult
noun
an·te·pe·nult ˌan-ti-ˈpē-ˌnəlt 
-pi-ˈnəlt
-pi-ˈnəlt variants or less commonly antepenultima
ˌan-ti-pi-ˈnəl-tə-mə
: the third syllable of a word counting from the end (such as cu in accumulate)
antepenultimate
ˌan-ti-pi-ˈnəl-tə-mət
adjective or noun 
Word History
Etymology
Late Latin antepaenultima, feminine of antepaenultimus preceding the next to last, from Latin ante- + paenultimus penultimate
First Known Use
circa 1620, in the meaning defined above