Hello watties,
first welcome to the forum and sorry that your fish is sick

Unfortunately the info you give us is not much, therefore a few questions:

how big is your tank?
when did you set up the tank?
when did you add your fish?
how many fish do you keep in there?
how is ammonia, nitrite, nitrate, pH, temp?
what kind of filtration and aeration do you have?
how much do you feed and what?
are all fins clamped or only the tail fin?
any other abnormal things you noticed?
can you post a pic?

Just for now I would test the waterquality and do a 20-30% waterchange.
Observe your fish how he/she behaves after you refilled with fresh and treated water

Shubunkin are known for being a skinny fish, it doesn't take much for them especially the males to go from healthy to starving.  It sounds like your fish is well on its way to starving to death and will no longer eat.  I once saved a lovely 6" shubunkin I bought from the pet shop, it was still swimming but was refusing to eat, I saved it by pushing soft wet pellets down its throat (past the teeth in its throat) after a week it started to feed on live water fleas and is now a top breeder.