"the science of the inward and essential nature of things," 1560s, plural of Middle English metaphisik, methaphesik (late 14c.), "branch of speculation which deals with the first causes of things," from Medieval Latin metaphysica, neuter plural of Medieval Greek (ta) metaphysika, from Greek ta meta ta physika "the (works) after the Physics," title of the 13 treatises which traditionally were arranged after those on physics and natural sciences in Aristotle's writings. See meta- + physics.
The name was given c.70 B.C.E. by Andronicus of Rhodes, and was a reference to the customary ordering of the books, but it was misinterpreted by Latin writers as meaning "the science of what is beyond the physical." The word originally was used in English in the singular; the plural form predominated after 17c., but singular made a comeback late 19c. in certain usages under German influence. From 17c. also sometimes "philosophy in general," especially "the philosophical study of the mind, psychology."
word-forming element making ordinal numbers (fourth, tenth, etc.), Old English -ða, from Proto-Germanic *-tha- (cognates: Gothic -da, -ta, Old High German -do, -to, Old Norse -di, -ti), from PIE *-to-, also *-eto-, *-oto-, suffix forming adjectives "marking the accomplishment of the notion of the base" [Watkins].
Cognate with Sanskrit thah, Greek -tos, Latin -tus; Sanskrit ta-, Lithuanian and Old Church Slavonic to, Greek to "the," Latin talis "such;" Greek tēlikos "so old, of such an age," Old Church Slavonic toli "so, to such a degree," toliku "so much," Russian toliko "only;" also see -ed.