Taos lightning was a slang term for a straight Earth bourbon of dubious origin and low quality.
After being injured by in a bar fight with Wyatt Earp, Doctor McCoy tended to Captain Kirk's wounds by dabbing them with Taos Lightning, much to Kirk's discomfort, referring to it as "fire." McCoy suggested the stuff to be "medicinal" in small amounts, to which Kirk replied that it should be labeled "for external use only."
Commander Scott, who was disappointed by the absence of Scotch in the Ancient West, learned to settle for Taos lightning, describing it as something that "takes a bit of getting used to", but that "a man could grow quite fond" of it.
Later, both Kirk and McCoy warned Scott to limit his consumption of the stuff. McCoy was concerned about what it might be doing to Scotty's stomach lining; Kirk was concerned what it would do to his reflexes (as they were about to be forced into a gunfight). (TOS: "Spectre of the Gun")