A probe casing was the outer shell of a probe.
The probe casing of a class 8 probe, which measured just over 2 meters in length, could be used as an impromptu warp vessel, capable of warp 9, following the removal of its transmitters and the sensors, and the installation of life support. In 2365, such a instance was experienced by Klingon special emissary K'Ehleyr, who upon her arrival aboard the USS Enterprise aboard such a probe, referred to it as a "damned coffin" after she endured a trip aboard it from Starbase 153 to her rendezvous point with the Enterprise. (TNG: "The Emissary")
While investigating the Hekaras Corridor in 2370, the Enterprise-D located what was described by Lieutenant Commander Data as "a small metallic object, approximately 3.5 meters in diameter." Commander William T. Riker theorized that it "could be a ship's log recorder, or a probe casing." Data's sensor readings indicated otherwise, determining it to be a verteron probe, that was disguised to be a signal marker. (TNG: "Force of Nature")
Later that year, Data was put on assignment to located a Federation deep space probe that went off course and crash landed on Barkon IV, on account of the fact that some of the material used in the probe's casing was radioactive. (TNG: "Thine Own Self")
In 2371, USS Voyager discovered a new stable transuranic element that had "an unusually large atomic mass, over 150 nucleons." B'Elanna Torres considered the possibilities of what could be done with such an element, including making "probe casings that could go into the core of a sun, ultra-thin reactor shielding." (VOY: "Emanations")
In 2372, the USS Defiant carried aboard two atmospheric probes, which Captain Benjamin Sisko asked if Chief Miles O'Brien could rig with warheads from quantum torpedoes. O'Brien confirmed that it could be done, if they pulled the imaging systems to make room in the probe casings, estimating that to do the job would require 20 minutes. Sisko gave him 10 to complete the task. (DS9: "Starship Down")