On a Latin Loan
Sicuro is originally a localized Italian adaptation of Latin Securo meaning "reliable, safe, certain, guaranteed, assured, secure".
It is often used casually with an equivalent meaning to English "of course".
Sicher, pronounced zee-sher, means the exact same thing as Sicuro, although it is a German word.
This word Sicuro is particularly interesting, because it can be found across Europe in some form, Swedish säker, Danish and Norwegian sikker, obviously the previously mentioned English word, Luxembourgish secher, Spanish siguro, Albanian sigur, and on and on.
Why is this?
Latin became the language of administration, of politics and religion, through the medium of the original Roman Empire, and the universal/catholic church that built on a framework of beliefs that the Romans adapted from the Greek sect of "followers of Christ" who in turn adapted their version from a faction of Hebrews that rebelled (ironically in part against the Romans who occupied Palestine) from Judaism.
There are certain words that would be in everyone's interests if it had the same meaning everywhere.
A word that means "guaranteed", would be one of those words.
It is also a word with economic, religious, and contractual uses, a merchant may guarantee a delivery, a priest may guarantee salvation, and a landowner may guarantee the rights to the use of a specific plot.
No surprise then that this word made its way across Europe maintaining the same meaning and basic sound.
Since I find names interesting, I'll mention for others that do as well, basically every single one of these variations of Sicuro is also a surname, which you can find typing or copy/pasting the variants into forebears.io. In Italy, you can find Sicuro in Puglia, Securo in Veneto, Sicuri in Emilia-Romagna, Sicher in Trentino despite the fact it is a German word (that's worth looking into, interesting backstory), Secher in Denmark as an adaptation of Sikker, Secher also in France although the vast majority of the instances in that country is more likely to be from a different Latin word meaning "dry, sober", and more.
As I may often do before thinking too much about the implications of a specific historical possibility, I consulted someone more educated than myself before writing on this subject; in this case, it was Anatoly Liberman of the University of Minnesota, who runs an interesting blog himself, Oxford Etymologist https://blog.oup.com/category/oxford_etymologist/