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The person, the object of a joke or crticism, who happens to overhear or watch while she's being made fun of or criticised by someone who is unaware of this person's, the object's, presence.

A term attributed to Jim Downy of SNL by Tina Fey in her book Bossypants.
The boss was a sneaker-upper standing behind Shiftless while the latter was doing an impression of the former telling everyone to "save money by transcribing invoices instead of wasting money on photocopies."
by harmess drudge May 11, 2011