Thamyris (and then by Apollo). He was the son of Amyclas, king of Sparta, and Diomede. His name is almost certainly not Greek, as the -inth element testifies.
However, his name is traditionally linked with the hyacinth, into which flower he turned, or rather his blood turned, when he was accidentally killed by a discus thrown by Apollo. The flower in the story is not our hyacinth but a type of iris. As with many plants, it was believed to have special properties, in this case its bulb was claimed to have the power of postponing a boy's puberty.
This may have a bearing for Hyacinthus, whose puberty was 'postponed' indefinitely. The origin of the flower name itself, however, is still unknown. This is therefore one name that we shall have to leave, alas, without any proposed derivation as such.
There is one nice little linguistic note that we can add, however, before altogether abandoning Hyacinthus: on the base of its petals the flower has marks that resemble the early Greek letters AI, and this is said to be either a cry of lamentation uttered by Apollo or else the first letters of the boy's name (in classical Greek it begins with the letter upsilon, y).