PWS

*taḥt- - under (HALOT 1721‒1722, CDG 573, Arakelova 2001:43, Kogan 2015:384)

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).

Ugaritic
tḥt - under, beneath (DUL 865)
Phoenician
tḥt - under (Tomback 1978:338)
Hebrew
taḥat - below, underneath; what is located undemeath (HALOT 1721)
Old Aramaic
tḥt - under; in the place of, instead of, in exchange for (DNWSI 1209)
Official Aramaic
tḥt - under; in the place of, instead of, in exchange for (DNWSI 1209)
Official Aramaic
tḥt - under; in the place of, instead of, in exchange for (DNWSI 1209)
Demotic Aramaic
tḥt - under; in the place of, instead of, in exchange for (DNWSI 1209)
Palmyrean
tḥt - under; in the place of, instead of, in exchange for (DNWSI 1210)
Nabataean
tḥt - under; in the place of, instead of, in exchange for (DNWSI 1210)
Syriac
tḥet - below, under (LSyr. 821, SL 1637)
Mandaic
atutia, tutia - beneath, under, underneath, below (MD 43)
with the loss of ḥ (Arakelova 2001:43)
Arabic
taḥta - beneath, below, under (Lane 298)
Sabaic
tḥt - beneath, below (SD 147)
Minaean
tḥt - sous (under) (LM 96)
Qatabanian
tḥthw - under, beneath (LIQ 177)
Geez
tāḥta - below, under (CDG 572)
Tigre
täḥat - downwards (WTS 305)
Tigrinya
təḥti - under, below, beneath, infra- (TED 1227)
Amharic
tačč - below, down there (AED 979)
Harari
taḥay - down, under (EDH 149)
Selti
tātä - down, under, beneath (EDG 604)
Wolane
tat - down, under, beneath (EDG 604)
Chaha
tät - down, under, beneath (EDG 604)
Ezha
tät - down, under, beneath (EDG 604)
Endegen
taˀaččä/taˀačče - down, under, beneath (EDG 604)
Ennemor
taˀačä - down, under, beneath (EDG 604)
Gyeto
tät - down, under, beneath (EDG 604)
Muher
tät - down, under, beneath (EDG 604)
Masqan
tätte - down, under, beneath (EDG 604)
Soddo
tatä - down, under, beneath (EDG 604)