Correct spelling for the English word "Mastful" is [mˈastfə͡l], [mˈastfəl], [m_ˈa_s_t_f_əl] (IPA phonetic alphabet).
Abounding in the fruit of the oak, beech, or chestnut.
Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.