The word 'twentieth' is a direct descendant of the Old English ordinal number 'twentigoþa', formed by combining the cardinal number 'twentig' (twenty) with the ordinal suffix '-oþa'. The cardinal 'twentig' itself originates from the Germanic root 'twai-' (two) combined with 'tig-' (group of ten), literally meaning "two tens." The ordinal suffix '-eth' is the modern reflex of the Old English '-oþa', which is cognate with similar ordinal suffixes across other Germanic languages. Thus, the literal morphemic construction "two-tens-th" logically evolved into the modern abstract meaning denoting position or fractional division in a series of twenty.

Structure: twenti (twenty) + eth (ordinal suffix) - twenti: From Old English 'twentig', meaning the number twenty. Its functional role is the cardinal base. - eth: From Old English '-oþa', '-aþa', an ordinal number suffix. Its functional role is to form ordinal numbers from cardinal bases.