To both masturbate and defecate simultaneously, usually on a lavatory or into a porcelain container.
Many begin to defecate before masturbating, so as to start an uninterrupted fecal flow. Also, it is not a requirement to ejaculate and finish defecating at the same time, but dedicated "shankers" will attempt the 'Snow on the Mountain' technique (wherein the discharge lands on the fecal matter.)
It is not necessary to be nude, but serial "shankers" will usually derobe before attempting a long shank.
Guy 1: "Jeez, you were ages man. What took you so long? Big shit?"
Guy 2: "Nah man, had a good shank."
Guy 1: "Well, thats me kicked out"
Guy 2: "What did you do?"
Guy 1: "My girlfriend woke up and found me shanking next to her last night."