STP is an acronym for stand-to-pee devices, which are designed for use by pre-operative female-to-male trans men, according to FTMGuide.org. These devices make it possible for FTM transgender men to urinate from a standing position while using men's restrooms.
STP devices are an essential tool for many FTM transgender men that allow them to use men's restrooms safely while in public. FTMGuide explains that many FTMs opt to use STP devices before obtaining sexual reassignment surgery, while others opt not to have surgery at all and rely on these devices instead. These devices can be made at home from inexpensive materials. There are also manufacturers that produce STP devices in a range of prices.
STP devices originated as an offshoot from female urination devices, which have been used since 1922, according to Wikipedia. These devices were originally intended for women who chose to urinate from a standing position for various reasons, and have been referenced in pop culture around Europe. STP devices were built on the FUD concept, but were adapted for transgender men as a way to keep their biological sex discrete in public. Due to discrimination towards transgender individuals in many parts of the world, FTMGuide.org notes that using an STP device is not just a matter of personal preference but a safety precaution for many men.