The term describes a state in which the penis becomes severely swollen and red after being bitten by a spider. It is usually accompanied by an uncomfortable itching sensation that, when taken care of, only worsens one's situation. Remedies are unknown, good luck.
Guy 1: What the hell! I think I have spiderpenis...
Guy 2: Woah, stay away from me! I dont want to catch it.
Guy 1: Its not contagious you dillhole. A spider bit my dick.