why x-rays are called x-rays???not y-rays.?

Answer 1

Wilhelm Röntgen called them "X-rays" and the name has been used ever since. He used "X" to signify that they were unknown, which they were at that time.

Wikipedia:
On November 8, 1895, Wilhelm Conrad Röntgen, a German physics professor, began observing and further documenting X-rays while experimenting with vacuum tubes. Röntgen, on December 28, 1895, wrote a preliminary report "On a new kind of ray: A preliminary communication". He submitted it to the Würzburg's Physical-Medical Society journal. This was the first formal and public recognition of the categorization of X-rays. Röntgen referred to the radiation as "X", to indicate that it was an unknown type of radiation. The name stuck, although (over Röntgen's great objections), many of his colleagues suggested calling them Röntgen rays. They are still referred to as such in many languages. Röntgen received the first Nobel Prize in Physics for his discovery.

Answer 2

when they were dicovered they were sort of unknown.

something unknown often gets the letter X
like planet X etc.

i think this state lasted very long with x-rays, and noone really wanted to call them Roentgen-Radiation.

so it was a mixture of lazyness and being confused about it.

Answer 3

X was used by Conrad as a symbol for something unknown. The term stuck.

Answer 4

you really should get out more!