I can’t believe the last time I did one of these was April. What took me so long?
Y is a weird letter. Not because of the whole sometimes-a-vowel-sometimes-a-consonant thing. It’s weird because while the sound has been around for a long time, the symbol for it has changed like a million times.
First of all, why do we pronounce it, well, “why”? No one knows. In Old English, the Y at the beginning of words like yard and yield was a throaty “gh” sound. The symbol wasn’t Y back then, but Ȝ, or yogh (pronounced “yokh”), an Old Irish letter that kind of combined G and Y. But then the French decided they weren’t going to use Yogh when they were transcribing in English anymore because waaaah! it wasn’t Latin! And in the early thirteenth century started using either Y or gh. Because of course they did.
So where’d the Y symbol come from? Good question, self (thanks, self). Up to T, our letters are all easily traced from Phoenician to Greek, Etruscan, and finally Latin. When the Romans got to it, they added V (which eventually also became U and then W) and X. Because they also took a lot of words from Greek, too, they added Y for Greek words they needed to spell. And they just straight up used the symbol from the Greek letter capital upsilon: Υ. They chose only capital upsilon because lower-case upsilon (υ) was too busy being the inspiration for U.
Tony Jebson’s page on the Origins of Old English