Squirt guns were a popular child's toy in the 20th century on Earth. They used pressure to eject water at others.
In 2294, Captain Hikaru Sulu and several members of his crew were taken hostage by a group of Beta Promethean traders led by Maldari. While the Federation was stymied, former Captain James T. Kirk and his command crew decided to take action to rescue the Excelsior hostages themselves.
Upon learning of the demands of the Promethan Captain Maldari to give them "powerful Federation weapons" Kirk decided the only way to effect the release of the hostages was to appear to give the Prometheans what they wanted. Gaining access to Starbase 499's replication facilities Kirk, Montgomery Scott, and Leonard McCoy replicated several cases of squirt guns from a 20th century toy catalog.
Kirk and his senior staff then traveled to the Beta Promethean star system on the Plush Princess to trade the "weapons" for the hostages. Upon beaming down Maldari attempted to double cross Kirk, however within minutes the water that was inside the squirt guns had frozen and the guns would not work. Before stunning the hapless Beta Promethean, Kirk informed Maldari that the weapons he had given them would not work when it was below freezing. Once back on board the Plush Princess Kirk admitted that even if it was above freezing the only thing that would've happened was that they would've gotten a bit wet.
Kirk later justified his actions, stating that the General Orders never mentioned toys. (TOS novel: The Fearful Summons)