A house of ill-fame, to which persons were enticed by women who act as decoys, and where, after making sure of the victim, a man - usually playing the assumed part of the woman's husband - suddenly enters the room by a panel or other secret passage and extorts money and valuables by threats of violence or, if the sufferer be asleep, robs him of whatever valuables he may possess.

Reportedly, one of the more famous victims of the use of a "panel room" or "panel house" was none other than Phineas T. Barnum who, having coined the phrase "there's a sucker born every minute," really should have known better. Staying with a woman "for company", he was accosted by a man claiming to be the woman's husband, who threatened to kill Barnum for the injustice done unless Barnum coughed up enough cash to settle the man's temper.

Apparently, no blood was shed.

Source: T. Zell's Popular Encyclopedia, 1871.