Perhaps a fossilized earlier genitive form of tuoi (“second”) (modern genitive form – tuoiz). Compare Estonian üksteist (“eleven”) a shortening of üksteistkümmend "one of the second ten" where "ten" gets dropped and "one of the second" remains.
tuoistõn
LĒL lists only a plural form (tuoistõnd). Both singular and plural are given in the declension table in this entry. Perhaps by error the declension type used for ordinals of teens (tuoistõnz (“teenth”)) is given in LĒL, evidently tuoistõn would have the same declension type as its simple compounds, e.g., kakštuoistõn, which is given above.
| singular (ikšlug) | plural (pǟgiņlug) | |
|---|---|---|
| nominative (nominatīv) | tuoistõn | tuoistõnd |
| genitive (genitīv) | tuoistõn | tuoistõnd |
| partitive (partitīv) | tuoistõnt | tuoistiņi |
| dative (datīv) | tuoistõnõn | tuoistõndõn |
| instrumental (instrumentāl) | tuoistõnõks | tuoistõndõks |
| illative (illatīv) | tuoistõnõ | tuoistiņiž |
| inessive (inesīv) | tuoistõnõs tuoistõns |
tuoistiņis |
| elative (elatīv) | tuoistõnõst tuoistõnst |
tuoistiņist |