early 14c., "a piece of cloth," especially a rectangular piece, from Old French panel "piece of cloth, piece, saddle cushion" (Modern French panneau), from Vulgar Latin *pannellus, diminutive of Latin pannus "piece of cloth" (see pane).
Anglo-French legalese sense of "piece of parchment (cloth) listing the names of those summoned to serve upon a jury" led by late 14c. to the meaning "a jury selected for a trial." General sense of "persons called on to advise, judge, discuss," etc. is from 1570s. Sense of "more or less distinct part of the surface of a wall, door, etc." is recorded from c. 1600.
Panel-house (said to be from 1840s; popular from 1870s) was old slang for a disreputable place (typically a bordello) with panneled rooms. At least one panel could be slid back to allow for thefts from customers and other cheats. Hence panel-thief, panel-game, etc.
The requisites for a "panel house" in the proper sense, are,—a crafty, cunning street walker; a not less cunning and at the same time sturdy scoundrel—known in the slang of the business as a "Badger," and a room prepared specially for the purpose by having a small invisible opening, generally a noiselessly opening panel in the partition or entrance door, by which access to the place can be had from an adjoining room. These three requisites obtained, it becomes the duty of the panel-thief to find the fourth in any "greenhorn" that can be picked up on the streets and induced to come into the apartment. ["The Dark Side of New York Life and its Criminal Classes," 1873]
1540s, "senior pupil at a school charged with keeping order, etc.," from Latin monitor "one who reminds, admonishes, or checks," also "an overseer, instructor, guide, teacher," agent noun from monere "to remind, bring to (one's) recollection, tell (of); admonish, advise, warn, instruct, teach," from PIE *moneie- "to make think of, remind" (source also of Sanskrit manayati "to honor, respect," Old Avestan manaiia- "making think"), suffixed (causative) form of root *men- (1) "to think" (source also of Latin memini "I remember, I am mindful of," mens "mind") The notion is "one who or that which warns of faults or informs of duties."
The type of lizard (1826) was so called because it is fabled to give warning to man of Nile crocodiles. Meaning "squat, slow-moving type of ironclad warship" (1862) is from the name of the first vessel of this design, chosen by the inventor, Swedish-born U.S. engineer John Ericsson (1803-1889), because it was meant to "admonish" the Confederate leaders in the U.S. Civil War.
I now submit for your approbation a name for the floating battery at Green Point. The impregnable and aggressive character of this structure will admonish the leaders of the Southern Rebellion that the batteries on the banks of their rivers will no longer present barriers to the entrance of the Union forces. The iron-clad intruder will thus prove a severe monitor to those leaders. ... "Downing Street" will hardly view with indifference this last "Yankee notion," this monitor. ... On these and many similar grounds I propose to name the new battery Monitor. [Ericsson to Asst. Sec. of Navy, Jan. 20, 1862]
Broadcasting sense of "a device to continuously check on the technical quality of a transmission" (1931) led to special sense of "a TV screen displaying the picture from a particular camera."