Whats a sub-epidermal cyst on penis?

Under skin of penis: In a prior answer on a similar topic, i mentioned that pain in the penis could be caused by an abnormality under the top layer of the skin = sub-epidermal. In your question, this abnormality is probably a benign cyst with fluid. See a urologist, may be able to drain it with very fine needle or remove cyst entirely (don't wince so much, local anesthesia will be used).