Scalosian water was water from the planet Scalos which, when imbibed, hyper-accelerated the drinker so that they moved so fast that they could not be seen by anyone who was not so accelerated. The acceleration was caused by radioactive contamination which, with long term exposure, caused sterility in males.
In 2268, James T. Kirk was hyperaccelerated without his consent by Deela, who put Scalosian water in his coffee. Doctor Leonard McCoy determined this and created a counteragent, which Spock proceeded to give to the captain by drinking the water himself and delivering it once in the same state. Kirk drank the counteragent and returned to normal, while Spock remained hyperaccelerated in order to repair damages wrought on the USS Enterprise by the Scalosian intruders. (TOS: "Wink of an Eye")