Y est littera greca et vocatur ab eis ypsilon.
ypsilon ABC | i.psilo f | i.piso e
The letter Y is Greek and is called ypsilon by them.
The Greek letter name Y ψιλόν /Y psilón/, which means literally: "bare-" or "simple U" seems to reflect an early distinction between two Greek letter shapes both derived from the same Semitic/ Phoenician letter waw. This ancestral form, shaped like a capital 'V', was indeed the letter from which Y and F derived. The "simpler" form Y was used for the vowel sound /u/ as in English "ooze", and the digamma - similar in shape to capital 'F' - for the consonant /w/ as at the beginning of "warm". This latter sound /w/ was however lost early on in most Greek dialects but survived in the Western Greek alphabet that was adopted by the Etruscans and the Romans and is in fact the ancestral form of our capital 'F'.
The letter Y appears in Classical Latin only in words of Greek origin. The sound represented by this letter in Greek underwent considerable changes in the evolution of Greek from archaic /u/ as in French 'ou', English 'oo', to a sound like German 'ü' or French 'u', to finally becoming /i/ as in French "midi", English 'ee'. These changes were reflected in the loans into Latin, which is why Greek Y in the pre-Classical period was often transliterated as 'u'. In classical times, where the educated classes routinely acquired some knowledge of Greek, the Greek letter form Y was used consistent with its occurrence in the original Greek words. At that time it was probably pronounced like German 'ü' or French 'u', although the hoi polloi seemed to have pronounced it /i/ as in French "midi", a pronunciation that became the norm in late and medieval Latin. Since 'I,i' and 'Y,y' now represented the same sound /i/ they were used quite interchangeably, e.g. Simon writes: Yris and Igia for Greek ἶρις /iris/ and ὑγιεια /(h)ygieia/. Cf. an illustration of the outlined sound changes can easily be seen in the plant name derived from Greek ἕρπυλλος /hérpyllos/: serpullum (Cato), serpyllum (Pliny}, serpillum (Simon), although these forms could and did sometimes exist simultaneously.