Usage: 2 forms in 6 verses
Form Translated As     In Verse(s)
τεσσαρακοστώ fortieth LXX.Num.33.38LXX.Deut.1.3LXX.1Chr.26.31LXX.2Chr.16.13
τεσσαρακοστόν fortieth LXX.Deut.8.2LXX.Josh.14.10

Liddell-Scott-Jones

τεσσαρακοστός (tessarakostos) fortieth G:A

τεσσαρακοστός-ός, , όν,

fortieth, [Refs 5th c.BC+]; Doric dialect τετρωκοστός, , όν, [Refs 3rd c.BC+]; also Ionic dialect apparently, [Refs 4th c.BC+]: but Ionic dialect Τετρηκοστή (proper name) [Refs]

__II τεσσαρακοστή (i.e. μοῖρα):
__II.1 tax of one-fortieth, [Refs 5th c.BC+]
__II.2 a fortieth, a coin of Chios, [Refs 5th c.BC+]