Origin of the word Y

1. a late-developing letter in Eng. Called ipsilon in Ger., upsilon in Gk., the Eng. name is of obscure origin. The sound at the beginning of yard, yes, yield, etc. is from O.E. words with initial g- as in got and y- as in yet, which were considered the same sound and often transcribed as a character that looks something like 3 (but with a flat top and lower on the line of text), known as yogh. The system was altered by Fr. scribes, who brought over the continental use of -g- and from the early 1200s used -y- and sometimes -gh- to replace 3. There's a good, in-depth discussion of yogh here. As short for YMCA, YWCA, YMHA first recorded 1915.

via Online Etymology Dictionary, ©2001 Douglas Harper