When you place a females underwear over you head and wank yourself off, usually with used underwear: the smellier the better.
Named so becuase one resembles spiderman with both eyes looking out through the leg holes.
Bob: I had a spiderman wank with your underwear last night, it fucking stank!
Sally: That's not my underwear, I'm helping old Mrs. Clegg with her laundry, she is 95 you know.
Bob: Ah well, I wondered why they smelled of fish AND cheese, but enjoyed it nonetheless.
Sally: I want a divorce.
When You Masterbate, Come In Your Hand And Throw It In Someones Face! SHOUT SPIDERMAN! ;')
I Will Spiderman Wank You In Your Sleep ;)