adv. once, formerly, at one time; already

Zeymahi lost ont du'ul Bormahu.
My brother was once the crown of our father.
(Paarthurnax, 0009C213)

Zu'u lost kriaan hi ont, nu hin sille fen nahkip suleyki.
I've killed you once, now your souls will feed my power.
(Alduin, 000ED365)

Het ont kriist Miraak, wo ahtiid ok sahvot ol Qah, spaan naal Deyra fah ok unslaad midun.
Here once stood Miraak, who wore his faith as Armor, shielded by the Daedra for his unending loyalty.
(Word Wall for Qah, Dragon Aspect)

Likely from English ‘once’. Used to mean ‘in the past’, not ‘one time’ (‘he struck once’) or ‘whenever’ (‘once he strikes, strike back’).