ysetin = once (abverb) (some things Google found for "ysetin": a rare term; name of a character on World of Warcraft; similar Vsetin is the name of places in the Czech Republic and Texas)
Word derivation for "once" :
Basque = behin, Finnish = yhdesti
Miresua = ysetin
I chose not to use the most common Finnish word for once, kerran. Kerran is related to kerta, which means time or occasion. Instead I decided to use yhdesti, which also means once, and resembles the Finnish word for the cardinal number one, yksi.
Usage notes on yhdesti from wiktionary: Although grammatically fully correct, yhdesti is, both in standard Finnish and in spoken Finnish, practically always replaced by saying (yhden) kerran – yhdesti is, however, quite normally used in mathematics.
The Basque word for once, behin, vaguely resembles the Basque word for the number one, bat. By the way, my Miresua word for one is yst.
I know that this word is a lopsided mix, with more letters from the Finnish word than the Basque word. But with the upcoming word for twice, it'll even out.
This Miresua conlang word has been changed. The word for once is now yhtin.