That would be its ancient Greek name.

It was one of several "i" sounding vowels (like english has: i as in bin, e as in be, y as in gym, etc, roughly sounding the same).

"Y" has "i psilon" meaning "high i" (literal) or "slim/bare i" (referring to how it was pronounced).

Psilon in this case means plain or simple (as opposed to compound or two-letter), as in spelled with one letter, because when the name was coined in the Koine Greek period, υ was pronounced /y/ like οι (but not like ι as in Modern Greek yet). Similarly, epsilon (ε) was pronounced like αι (as it still is).

In Estonia it's both Igrek and Üpsilon. That said, the letter is not in Estonian alphabet and is considered a "foreign letter".

Also in Portuguese!