No convincing
internal etymological reconstruction suggests itself, although the similarity
between *taḥta and the Proto-Aramaic root
*nḥt ‘to go down’ is noteworthy. It is usually assumed that this preposition is derived from the noun ‘lower part, what is beneath’ (DNWSI 1210, CDG 573), which is not unquestionable: at least, the distribution of the preposition over languages is much broader than that of the noun (Arakelova 2001:44).