Being interrupted mid-masturbation. Can be when one is directly caught in the act or is interrupted by other means, i.e. an unexpected guest at the door, a phone call, someone coming to check up on them, etc.
Similar to a cockblocker, the shockblocker is reserved for those attempting to block a lesbian hook-up. Although this hook-up may not really involve the shocker (two in the pink, one in the stink)... it will definitely involve at least two fingers and not a cock, and so shockblocker is the most appropriate term.